Update docs on git submodules

The docs currently do not reflect that:

* git submodules are init/updated automatically if there's a `.gitmodules` file
* the automatic integration can be turned off by setting `submodules: false`
Fix wording
Update build-configuration.md
Update build-configuration.md
Merge pull request #4 from travis-ci/sf-update-git-submodule-docs

Update docs on git submodules