Add new CONTRIBUTING.md
Various and sundry text updates to CONTRIBUTING.md

Namely, all sentences should end with periods. Other light changes.
Please cover the rest of my comments mentioned on commit 30fec1af (or
argue with them) then PR away!
Merge remote-tracking branch 'origin/pr/4' into feature/contributing
Add note about reporting vulns
Add an example of a good PR
Add missing period
Add more examples of good contributions
Change HTTP link to HTTPS

Doesn't redirect by default.
Add note about PR titles
Add note about the 50/72 rule
Add note about topic branches
Add note about descriptive titles in bug reports
Add a link to common coding mistakes
Update HACKING along with CONTRIBUTING.md
Merge remote-tracking branch 'origin/pr/5' into feature/contributing
Land #3063, make CONTRIBUTING and HACKING useful