Add instructions on sbt options in developer docs

[Sumith1896]

Information as to how to generate docs locally and other sbt options need to be added to the developer docs.

As of now I couldn't find this anywhere, is it documented?

[d6y]

There's a page here: https://github.com/slick/slick/blob/master/slick/src/sphinx/sphinx-documentation.rst

...but that's not yet published on the docs web site. I presume it will be for the next release.

Also: https://gitter.im/slick/slick?at=56e1c4aa0055f8f35a820831