add contributing

CONTRIBUTING.md

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+# Contributing
+
+Hi!  I just wanted to say, if you're thinking of contributing to `aws-sso`
+I'm excited to have all the help I can get. :)  That said, having worked
+on this for a few years I've definitely got some opinions regarding how
+things should work.  Some of those opinions are certainly right and others
+are undoubtably wrong.  The trick is of course figuring out which is
+which.
+
+With that said, please reach out before you start working on a new
+feature so we can discuss.  Easiest way is to 
+[open a feature request](
+https://github.com/synfinatic/aws-sso-cli/issues/new?assignees=&labels=enhancement&projects=&template=feature_request.md&title=)
+and start a conversation.
+
+Thanks!