Update CONTRIBUTING.md

Feature requests should be discussed over the core mailing list.
Remove old mix artifacts without a need for force or changes
always remove unmatched beam files when compiling
Fixed problem with elixir.bat removing exclamation marks from command line
Merge pull request #1517 from ToJans/master

Fixed problem with elixir.bat removing exclamation marks from command line
Fixing doc typos in File
Add &local/arity and &Remote.fun/arity
Deprecate function/1 and function/3
Merge pull request #1519 from jwarwick/file_typos

Fixing doc typos in File
Allow &() as a general capture mechanism
Fix Macro.to_string with &
Allow &1.foo syntax inside capture
Fix Stream bugs and add more composing tests

Fix Stream.take_while looping infinitely when given lazy stream.
Fix Stream.take and Stream.take_while calling fun one extra time.
Add more tests where Stream functions are given streams.
Merge pull request #1521 from ericmj/stream-fixes

Fix Stream bugs and add more composing tests
Normalize and allow captures in Regex.scan/3, closes #1526
Fixed doc typos in Regex
Merge pull request #1528 from jwarwick/regex_typos

Fixed doc typos in Regex
Fix issue with nested binaries in Macro.to_string/1, closes #1522
Fixed doc typos in Macro
Update docs for Mix.project/0