Explain contributing.md

[danirabbit]

Problem

Currently we have some project specific quirks in the contributor guide, but this is sometimes duplicate information and better kept in CONTRIBUTING.md files in each project to stay current. GitHub also surfaces contributing information when it's present, so there's some incentive to have it there in the repo

Proposal

We should move the current quirk info into CONTRIBUTING files on the relevant repos and instead add a section to the intro about looking for CONTRIBUTING.md files on the project repo

Prior Art (Optional)

https://github.com/elementary/icons/blob/master/CONTRIBUTING.md