Add initial CONTRIBUTING.md

CONTRIBUTING.md

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+Contributing to Idris 2
+=======================
+
+Contributions are welcome! The most important things needed at this stage,
+beyond work on the language core, are (in no particular order):
+
+* CI integration
+* Documentation string support (at the REPL and IDE mode)
+* A better REPL, including:
+  - readline and tab completion
+  - :search and :apropos
+  - help commands
+* Further library support (please add initially into contrib/)
+
+Some language extensions from Idris 1 have not yet been implemented. Most
+notably:
+
+* Type providers
+  - Perhaps consider safety - do we allow arbitrary IO operations, or is
+    there a way to restrict them so that they can't, for example, delete
+    files or run executables.
+* Elaborator reflection
+  - This will necessarily be slightly different from Idris 1 since the
+    elaborator works differently. It would also be nice to extend it with
+    libraries and perhaps syntax for deriving implementations of interfaces.
+
+Other contributions are also welcome, but I (@edwinb) will need to be
+confident that I'll be able to maintain them!
+
+Some syntax that hasn't yet been implemented but will be:
+
+* Range syntax (e.g. [1..10], [1,3..10], [1..] etc) [needs some thought about
+  what should go in the Prelude and exactly how to implement]
+* Idiom brackets
+* !-notation [needs some thought about the exact rules]
+
+Some things from Idris 1 definitely won't be implemented:
+
+* Uniqueness types (instead, Idris 2 is based on QTT and supports linearity)
+* Some esoteric syntax experiments, such as match applications