Add CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to App::GitGot
+
+We appreciate your desire to contribute to GitGot.
+
+Please note, App::GitGot is stable, mature software that many people
+depend on for their day-to-day workflows. Any contribution needs to
+either fix an outstanding bug, or to add a useful new feature to the
+project. In particular, changes that are "lateral upgrades" of
+dependencies are unlikely to be accepted. Changes that alter existing
+interfaces will _NOT_ be accepted.
+
+The ideal workflow is to open a new issue proposing your feature, or
+describing the bug you'd like to fix, along with a rough outline of
+how you propose to implement your change. Doing that prior to starting
+to code will likely save time and frustration for all.