Add contribution guidelines.

Author: thisandagain

CONTRIBUTING.md

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+## Introduction
+
+Individuals making significant and valuable contributions are given commit-access to the project to contribute as they see fit. In that way, this project is more like a wiki than a standard guarded open source project.
+
+## Rules
+
+There are a few basic ground-rules for contributors:
+
+1. **No `--force` pushes** or modifying the Git history in any way.
+1. **Non-master branches** ought to be used for ongoing work.
+1. **External API changes and significant modifications** ought to be subject to an **internal pull-request** to solicit feedback from other contributors.
+1. Internal pull-requests to solicit feedback are *encouraged* for any other non-trivial contribution but left to the discretion of the contributor.
+1. Contributors should attempt to adhere to the prevailing code-style.
+
+## Releases
+
+Declaring formal releases remains the prerogative of the project maintainer.