fix typo in CONTRIBUTING.md
Update Wall of Browser Bugs entry for #14972

http://crbug.com/429604 has been merged into http://crbug.com/175502
Document Savage cross-browser JS pull request tester bot

Refs #12352, which is now implemented & deployed.
More info: https://github.com/twbs/savage

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Merge pull request #15187 from twbs/doc-savage-contributing

Document Savage cross-browser JS pull request tester bot
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