Fix link to Pull Request help in CONTRIBUTING.md

GitHub changed their help page URLs and did not correctly redirect the
old Pull Request help URL.
Corrected terminology in comment

Really, really nitpicky change here but ... the comment was referring to the selection rule sets as "declarations", which isn't exactly the correct term. See:

http://nimbupani.com/css-vocabulary.html
http://www.impressivewebs.com/css-terms-definitions/
Merge pull request #1313 from impressivewebs/patch-1

Corrected terminology in comment
Fixed typo in `main.css`
Fix 404 widget script URL

Fix 404 widget script URL to be compatible with HTTPS by replacing
`http://` with `//`. The script hosted by Google is available on both
protocols.
Update CHANGELOG