Moban.rst: Doc on Moban Usage in Coala

[Lancasterwu]

This PR provides documentation of Moban usage, what it is and how Coala used it.

This is related to Issue #5621 in Coala.

Documentation on Moban usage is important especially when Moban is currently failing Travis CI build of Coala, related to its own upstream problem. See Issue #363 in Moban.

For short term contributors: we understand that getting your commits well defined like we require is a hard task and takes some learning. If you look to help without wanting to contribute long term there's no need for you to learn this. Just drop us a message and we'll take care of brushing up your stuff for merge!

Checklist

• [x] I read the commit guidelines and I've followed them.
• [x] I ran coala over my code locally. (All commits have to pass individually. It is not sufficient to have "fixup commits" on your PR, our bot will still report the issues for the previous commit.) You will likely receive a lot of bot comments and build failures if coala does not pass on every single commit!

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