Add link to contributing docs in CONTRIBUTING.md

[dr-rompecabezas]

As a new contributor, starting from GitHub, it took me a couple of extra steps to find the detailed contributing guidelines in the documentation.

I suggest we link to the docs directly from CONTRIBUTING.md.

[tim-schilling]

Hmm, I wonder if we should drop that top level file instead.

[matthiask]

CONTRIBUTING is one of the files recommended here:
https://github.com/django-commons/django-debug-toolbar/community

We don't necessarily have to bow to our GitHub overlords but keeping the file around may be useful.

Linking the docs sounds like a good idea!

[blingblin-g]

I'm interested in this issue. Could you please assign it to me?

[matthiask]

@blingblin-g Sure, thank you!