Update dead links in .github/CONTRIBUTING.md:

[whytrall]

Hi guys! I noticed that some links in CONTRIBUTING.md are no longer working:

• "this license agreement" link points to 404.
• "reduced test case" also points to nowhere
• "validate your HTML" link is dead

This is important, because new issue form has got a link to this file in an alert.

Thanks <З (hope you haven't forgotten about docs)

[broccolini]

Hi @trall1488 thanks for letting us know about those issues. We'll get those fixed.

hope you haven't forgotten about docs

Nope! We make documentation updates in most releases :)

Hi guys!

Please use gender neutral terms such as "folks", "everyone", "y'all" etc. 🙃.

[whytrall]

documentation updates in most releases :)

I mean something like primer.github.io/archive. It's much easier to use than to search through folders.

Please use gender neutral terms such as "folks", "everyone", "y'all" etc. 🙃.

English isn't my native language and I thought "guys" is neutral. There's always something new to learn. Thanks (and I'm sorry)!

[whytrall]

Closed #452