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.

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