First stab at a CONTRIBUTING.md file

CONTRIBUTING.md

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+# Contributing to Babel
+
+Babel is open-source software and all contributions are very welcome!
+
+## Reporting bugs
+
+Reporting when you see something wrong with Babel is very helpful, whether you spot it in the
+Babel output files or through one of the frontends. Following these guidelines will help you
+submit the most useful bug reports and will help us triage and prioritize them correctly.
+
+1. Is the issue related to the [Node Normalizer] application, such as invalid output, an unexpected
+   error message, mishandling input, or something else? If so, please report them in the
+   [Node Normalizer issue tracker](https://github.com/TranslatorSRI/NodeNormalization/issues/).
+2. Is the issue related to the [Name Resolver] application, such as invalid output, an unexpected
+   error message, mishandling input, or search results not being ranked correctly? If so, please
+   report them in the [Name Resolver issue tracker](https://github.com/TranslatorSRI/NameResolution/issues/).
+3. Most other issues will be [Babel issues](https://github.com/TranslatorSRI/Babel/issues/), in particular
+   anything to do with cliques whose identifiers, Biolink type, preferred label, other labels, synonyms or
+   descriptions are incorrect. If you're not sure about which repository your issue should go to, please
+   add it to Babel and we'll sort it out at our end.
+4. When reporting the bug, providing a link to the NodeNorm clique or NameRes query that shows the incorrect
+   output will be very helpful. We would also appreciate if you can include what you expect the tool to return.
+5. After you have reported a bug, helping to triage, prioritize and group it will be very helpful:
+    - We triage issues into one of the [milestones](https://github.com/TranslatorSRI/Babel/milestones):
+      - [Needs investigation](https://github.com/TranslatorSRI/Babel/milestone/12) refers to issues that need
+        to be investigated further -- either to figure out what is causing the issue or to communicate with the
+        user community to understand what should occur.
+      - [Immediate](https://github.com/TranslatorSRI/Babel/milestone/35) need to be fixed immediately. Issues I'm
+        currently working on will be placed here.
+      - [Needed soon](https://github.com/TranslatorSRI/Babel/milestone/30) refers to issues that should be fixed
+        in the next few months: not immediately, but sooner rather than later.
+      - [Needed later](https://github.com/TranslatorSRI/Babel/milestone/31) refers to issues that should be fixed
+        eventually, but are not needed immediately.
+      - [Needs testing](https://github.com/TranslatorSRI/Babel/milestone/27) refers to issues that need additional
+        testing, such as determining how widely an issue occurs, whether an issue is reproducible or -- in some cases --
+        to test whether a particular PR has in fact fixed the submitted issue.
+    - We prioritize issues with one of the three priority tags: [Priority: Low](https://github.com/TranslatorSRI/Babel/issues?q=is%3Aissue%20state%3Aopen%20label%3A%22Priority%3A%20Low%22),
+      [Priority: Medium](https://github.com/TranslatorSRI/Babel/issues?q=is%3Aissue%20state%3Aopen%20label%3A%22Priority%3A%20Medium%22),
+      [Priority: High](https://github.com/TranslatorSRI/Babel/issues?q=is%3Aissue%20state%3Aopen%20label%3A%22Priority%3A%20High%22).
+      The idea is that issues with the highest priority will determine which will be investigated/tested first, and which
+      are most likely to move from Needed later/Needed soon into Immediate for working on.
+    - You can group issues in two ways:
+      - GitHub lets you chose a "parent" issue for each issue, which is useful for issues that are related to each
+        other. We try to build "issues of issues" that group together similar issues that might require similar fixes
+        (e.g. [our issue tracking deprecated identifiers](https://github.com/TranslatorSRI/Babel/issues/93)). If you
+        find an issue related to yours, please feel free to add yours as a child of the existing issue or vice versa.
+      - You can use labels to group similar issues. We don't have a lot of labels for you to choose from, but feel free
+        to add any that make sense!
+
+## Writing code
+
+Babel is structured around its [Snakemake files](./src/snakefiles), which call into its
+[data handlers](./src/datahandlers) and [compendia creators](./src/createcompendia). The
+heart of its data are its concord files, which contain cross-references between different
+databases. These are combined into compendium files and synonyms.
+
+### Writing a new concord or compendium
+TBD
+
+### Adding a new source of identifiers, synonyms or descriptions 
+TBD
+
+### Submitting a PR
+TBD
+
+## Want to work on the frontends instead?
+
+Babel has two frontends: the [Node Normalizer] for
+exposing information about cliques, and the [Name Resolver],
+which lets you search by synonyms or names.
+
+- [Node Normalizer]: https://github.com/TranslatorSRI/NodeNormalization
+- [Name Resolver]: https://github.com/TranslatorSRI/NameResolution