Travis merge documentation (rebased onto develop)



This is the same as gh-477 but rebased onto develop.

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This PR adds a section about the `scc travis-merge` command to the developers section of the documentation. More precisely, this describes the way the travis-merge command parses the PR comments to construct the merge filter and merge PRs in the Travis bolds. 

Additionally, the filter types in the `scc merge` command section are reviewed, exposing both signature of the PR filter and exposing the signature to include/exclude submodule PRs by number.

/cc @ximenesuk