Add contributing guidelines (borrowed from metabrainz/picard)

[jwflory]

Summary

Adds basic contributing guidelines (mostly stolen from Picard a la @samj1912 ¹ ) to help newcomers contribute to ListenBrainz.

Background

I saw Picard's contributing guidelines and took those for inspiration. I added some of my own changes and additions.

Details

I tried to make this guide comprehensive for ListenBrainz. Feedback from @mayhem and @paramsingh especially welcome!

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¹ @samj1912 did awesome work on the Picard contributing guidelines, seriously! They were so easy to copy and reuse. 😄 🎉 💯
² The Travis CI build is failing because of metabrainz/brainzutils-python#6, I think.

[paramsingh]

Should we include something about writing tests for (ideally) each bugfix or new feature added?

[jwflory]

@paramsingh Is there anything specific you want to mention there? Otherwise, I can try to add that in.

[paramsingh]

Just a small paragraph saying something like "Try to include tests in each new pull request for bugfixes or new features. Unit tests are present alongside their respective modules and integration tests are present in the directory listenbrainz/tests/integration".

[jwflory]

@paramsingh Done! Excerpted from the commit:

Writing unit tests

Unit tests are an important part of ListenBrainz. It helps make it easier for
developers to test changes and help prevent easily avoidable mistakes later on.

New bugfixes or new features should include unit tests. Unit tests are present
with their modules while integration tests are present in
listenbrainz/tests/integration. If you need help with writing a unit test, ask
in IRC or Discourse (links above).