Merge pull request #290 from getsentry/contributing

add CONTRIBUTING.md

Author: mitchellhenke

CONTRIBUTING.md

@@ -0,0 +1,24 @@
+## Contributing
+
+Contributions to Sentry Elixir are encouraged and welcome from everyone.
+
+Before submitting code, please run the test suite, format code according to Elixir's
+code formatter and [Credo](https://github.com/rrrene/credo). This can be done with
+`mix test`, `mix format` and `mix credo`.
+
+The build server will also run [dialyzer](http://erlang.org/doc/man/dialyzer.html)
+using [dialyxir](https://github.com/jeremyjh/dialyxir) to check the typespecs, but this can be onerous
+to install and run. It is okay to submit changes without running it, but can
+be run with `mix dialyzer` if you would like to run them yourself.
+
+Once all checks are passing, you are ready to [open a pull request](https://help.github.com/articles/using-pull-requests/).
+
+### Reviewing changes
+
+Once a pull request is sent, the changes will be reviewed.
+
+If any changes are necessary, maintainers will leave comments requesting changes
+to the code. This does not guarantee a pull request will be accepted, as it will
+be reviewed following each change.
+
+Once the code is approved, your changes will be merged!