Update documentation to clarify bats-assert loading

[jhart-r7]

In this documentation there was some inconsistency with regards to which bats supporting libraries were being referred to. The result was that if you followed this verbatim for the common github submodule case, many of the asserts wouldn't work and it might not be immediately obvious why. This PR updates the documentation to be more consistent in this regard -- now both bats-support and bats-assert are both always referenced.

[jasonkarns]

I think we should probably even consider replacing git submodules with git subtrees* as the recommended installation option (as subtrees require a lot less management bookkeeping than submodules.)

in addition to this PR, not instead of

[jhart-r7]

Yes, in reading up on subtrees I agree that might be a good alternative in the future, but for now (to your point) I think this PR would be helpful to land.