Document Savage cross-browser JS pull request tester bot

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

[skip sauce]

CONTRIBUTING.md

@@ -199,13 +199,15 @@ included in the project:
 **IMPORTANT**: By submitting a patch, you agree to allow the project owners to
 license your work under the terms of the [MIT License](LICENSE.md).
 
-### Pull request checker bot: Rorschach
+### Pull request bots
 
 [@twbs-rorschach](https://github.com/twbs-rorschach) is a Bootstrap bot that hangs out in our GitHub issue tracker and automatically checks all pull requests for a few simple common mistakes. It's possible that Rorschach might leave a comment on your pull request and then close it. If that happens, simply fix the problem(s) mentioned in the comment (there should be link(s) in the comment explaining the problem(s) in detail) and then either:
 
 * Push the revised version to your pull request's branch and post a comment on the pull request saying that you've fixed the problem(s). One of the Bootstrap Core Team members will then come along and reopen your pull request.
 * Or you can just open a new pull request for your revised version.
 
+[@twbs-savage](https://github.com/twbs-savage) is a Bootstrap bot that automatically runs cross-browser tests (via [Sauce](https://saucelabs.com) and Travis CI) on JavaScript pull requests. Savage will leave a comment on pull requests stating whether cross-browser JS tests passed or failed, with a link to the full Travis build details. If your pull request fails, check the Travis log to see which browser + OS combinations failed. Each browser test in the Travis log includes a link to a Sauce page with details about the test. On those details pages, you can watch a screencast of the test run to see exactly which unit tests failed.
+
 
 ## Code guidelines