Added contributing file

CONTRIBUTING.md

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+Contributing to SLY
+===================
+
+SLY, like most projects related to parser generators, is a niche
+project.  Although it appears to be a somewhat "new" project, it is
+actually an outgrowth of the PLY project which has been around since
+2001.  Contributions of most kinds that make it better are
+welcome--this includes code, documentation, examples, and feature
+requests.
+
+There aren't too many formal guidelines.  If submitting a bug report,
+any information that helps to reproduce the problem will be handy.  If
+submitting a pull request, try to make sure that SLY's test suite
+still passes. Even if that's not the case though, that's okay--a
+failed test might be something very minor that can fixed up after a
+merge.
+
+Project Scope
+-------------
+
+It is not my goal to turn SLY into a gigantic parsing framework with
+every possible feature.  What you see here is pretty much what it is--a
+basic LALR(1) parser generator and tokenizer.  If you've built something
+useful that uses SLY or builds upon it, it's probably better served by
+its own repository. Feel free to submit a pull request to the SLY README
+file that includes a link to your project.
+
+The SLY "Community" (or lack thereof)
+-------------------------------------
+
+As noted, parser generator tools are a highly niche area.  It is
+important to emphasize that SLY is a very much a side-project for
+me. No funding is received for this work.  I also run a business and
+have a family with kids.  These things have higher priority. As such,
+there may be periods in which little activity is made on pull
+requests, issues, and other development matters.  Sometimes you might
+only see a flurry of activity around the times when I use SLY in
+a compilers course that I teach.   Do not mistake "inaction" for
+"disinterest."  I am definitely interested in improving SLY--it's
+just not practical for me to give it my undivided attention. 
+
+Important Note
+--------------
+As a general rule, pull requests related to third-party tooling (IDEs,
+type-checkers, linters, code formatters, etc.) will not be accepted.
+If you think something should be changed/improved in this regard,
+please submit an issue instead.
+