Add docs for would-be contributors. (#213)

Author: reventlov

doc/contributing.md

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+# Contributing to Emboss
+
+If you would like to fix a bug or add a new feature to Emboss, great!  This
+document is intended to help you understand the procedure, so that your change
+can land in the main Emboss repository.
+
+You do not have to take a change all the way from start to finish, either: if
+you can get a design approved, then someone else can implement it much more
+easily.  Conversely, if you are looking for a way to help, you might look for
+existing [feature
+requests](https://github.com/google/emboss/labels/enhancement) that have
+designs or at [open design sketches](design_docs/) that you might be able to
+implement.
+
+
+## All Changes
+
+Because Emboss is a Google project, in order to submit code you will need to
+sign a [Google Contributor License Agreement
+(CLA)](https://cla.developers.google.com/).
+
+**IMPORTANT**: if your contribution includes code that is not covered by a
+Google CLA and is not owned by Google, the Emboss project has to follow special
+procedures to include it.  Please let us know ([filing an issue on
+GitHub](https://github.com/google/emboss/issues/new) is probably the easiest
+way) so that we can walk you through the process.  In particular, we generally
+cannot accept any code from StackExchange or similar sites, and any code that
+comes from a non-Google open source project needs to have an acceptable license
+and be committed to the Emboss repository in a specific location.
+
+
+### How-To Guides
+
+This document covers the process of getting a change into the main Emboss
+repository — i.e., what you need to do to get your change into
+[the main Emboss repository](https://github.com/google/emboss/).
+
+[How to Implement Changes to Emboss](how-to-implement.md) has an overview of
+how to make code changes to Emboss.
+
+[How to Design Features for Emboss](how-to-design.md) has an overview of what
+to think about during your design.
+
+
+### Bug Fixes vs New Features
+
+The general process for bug fixes and new features is the same, but bug fixes
+usually require less design work, and therefore can go through lighter
+processes.
+
+
+## Very Small Changes
+
+If you have a tiny change — for example,  making a fix that does not change the
+design of `embossc` — you can jump directly into coding.
+
+This process works best if your change is small and not likely to be
+controversial, or if you have already completed [the steps for small
+changes](#small-changes).
+
+1.  [Code up your proposed change](how-to-implement.md) and open a [pull
+    request
+    (PR)](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/creating-a-pull-request)
+    to [the main Emboss repository](https://github.com/google/emboss/).  If you
+    have not completed [the steps for small changes](#small-changes), this
+    gives the Emboss maintainers something more concrete to look at, but you
+    may end up doing more work if your initial proposal turns out to be the
+    wrong approach.
+
+2.  The Emboss maintainers will review your PR, and may request changes.  Don't
+    be discouraged if your PR is not immediately accepted — even PRs that
+    maintainers send to each other often have requests for changes!  We want
+    the Emboss code to be high-quality, which means helping you make your code
+    better.
+
+3.  Once your PR reaches a point where it is good enough, an Emboss maintainer
+    will merge it into the Emboss repository.
+
+
+## Small Changes
+
+If your change is small, but still requires some design work — for example,
+adding a new utility function in the C++ runtime library, or making a bug fix
+that involves re-structuring some of the `embossc` code — it is usually best to
+get some feedback before you start coding.
+
+1.  [File an issue on GitHub](https://github.com/google/emboss/issues/new), if
+    there is not an issue already.  It is best to use the *problem you want to
+    solve* for the issue title and description, and then propose your design in
+    a comment.
+
+2.  Once the Emboss maintainers have had a chance to review your proposal and
+    agree on the general outline, follow [the procedure for very small
+    changes](#very-small-changes).
+
+
+## Medium and Large Changes
+
+If you have a medium or large change — for example, introducing a new pass in
+`embossc`, adding a new data type, adding a new operator to the Emboss
+expression language, making a cross-cutting refactoring of `embossc`, etc. —
+then you should start by writing a *design sketch*.
+
+A design sketch is, basically, an informal design doc — it covers the topics
+that a design doc would cover, but may have open questions or alternatives that
+haven't been locked down.
+
+1.  [File an issue on GitHub](https://github.com/google/emboss/issues/new), if
+    there is not an issue already.  It is best to use the *problem you want to
+    solve* for the issue title and description.
+
+2.  Look at [existing design sketches](design_docs/) and [archived design docs
+    for changes that have already landed](design_docs/archive/) to get a feel
+    for what should be in a design sketch.
+
+3.  If you have not already done so, read [How to Design Features for
+    Emboss](how-to-design.md).
+
+4.  Write a draft design sketch for your change, and open a pull request
+    against [the main Emboss repository](https://github.com/google/emboss/).
+
+5.  It is very likely that your design sketch will need revision before it is
+    accepted.  If it does, do not be discouraged — we want your change to
+    succeed!
+
+6.  Once your design sketch has been accepted, you can move on to
+    implementation, following (more or less) the same procedure you would
+    follow for [small](#small-changes) or [very small
+    changes](#very-small-changes).  Depending on the complexity of the change,
+    you may need to split your implementation into multiple changes.