Document contributor's guide.

[arxpoetica]

From Gitter convo with Rich:

things like 'how to write tests and where to put them' would probably help people when reporting bugs (since PRs with failing tests are gold, second only to PRs with tests and fixes) and the test suite is a bit wild and woolly. lots of assumed knowledge in there, like how to 'solo' a test, and knowing to use snapshot tests sparingly

[arxpoetica]

@asweingarten threw down some awesome thoughts on this, which should give me a leg up while writing this:
https://drive.google.com/file/d/1A6PDHohwHsNwZk0udFU7JYStoK5uMZdb/view
https://gitter.im/sveltejs/svelte?at=5a58e8525a9ebe4f757c3a76

See also his work on #1083 while writing up his thoughts. And the PR: #1099

[antony]

@arxpoetica is this a duplicate / the same issue as #3791 ?

[arxpoetica]

@antony similar but different. However, it looks like @tanhauhau did this in #3765

WOOHOO!!!

Closing.