Link CONTRIBUTING.md in more places

[RedCMD]

Describe the enhancement

Should link the CONTRIBUTING.md file in more places
as its causing many invalid PR's

some few ~hundred or so
https://github.com/github-linguist/linguist/pulls?page=1&q=is%3Apr+is%3Aclosed+is%3Aunmerged+in%3Atitle+add+language

[lildude]

I doubt that will help much as people aren't reading the PR template either, but I'll give it a try.

I'm also not sure where else we can link from that would be more visible. It's already in the README (twice!!), repo sidebar and issue template.

[RedCMD]

maybe?