Add documentation to bring taskcluster-tools closer to the Github rec…

…ommended community standards

Added licensing info (MPL2), contribution suggestions, and a link to the Mozilla Community Participation Guidelines.

Addresses https://bugzil.la/1408073 for this repo.

Carrying-over review from taskcluster/taskcluster-tools#546

CODE_OF_CONDUCT.md

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+# Mozilla Community Participation Guidelines
+
+The most recent version of the Mozilla Community Participation Guideline can always be found here: https://www.mozilla.org/en-US/about/governance/policies/participation/

CONTRIBUTING.md

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+# How to Contribute
+
+We welcome pull requests from everyone. We do expect everyone to adhere to the [Mozilla Community Participation Guidelines][participation].
+
+If you're trying to figure out what to work on, here are some places to find suitable projects: 
+* [Good first bugs][goodfirstbug]: these are scoped to make it easy for first-time contributors to get their feet wet with Taskcluster code.
+* [Mentored bugs][bugsahoy]: these are slightly more involved projects that may require insight or guidance from someone on the Taskcluster team.
+* [Full list of open issues][issues]: everything else
+
+If the project you're interested in working on isn't covered by a bug or issue, or you're unsure about how to proceed on an existing issue, it's a good idea to talk to someone on the Taskcluster team before you go too far down a particular path. You can find us in the #taskcluster channel on [Mozilla's IRC server][irc] to discuss. You can also simply add a comment to the issue or bug.
+
+Once you've found an issue to work on and written a patch, submit a pull request. Some things that will increase the chance that your pull request is accepted:
+
+* Follow our [best practices][bestpractices].
+* This includes [writing or updating tests][testing].
+* Write a [good commit message][commit].
+
+Welcome to the team!
+
+[participation]: https://www.mozilla.org/en-US/about/governance/policies/participation/
+[issues]: ../../issues
+[bugsahoy]: https://www.joshmatthews.net/bugsahoy/?taskcluster=1
+[goodfirstbug]: http://www.joshmatthews.net/bugsahoy/?taskcluster=1&simple=1
+[irc]: https://wiki.mozilla.org/IRC
+[bestpractices]: https://docs.taskcluster.net/docs/manual/design/devel/best-practices
+[testing]: https://docs.taskcluster.net/docs/manual/design/devel/best-practices/testing
+[commit]: https://docs.taskcluster.net/docs/manual/design/devel/best-practices/commits
+