Extend Tags docs to include the tags SettingsKey

[sebastiaansamyn-tc]

In our project we have a bunch of tests that can run in parallel and a whole bunch that cannot. To split these out we've created a custom scope and ran them with parallelExecution := false.

This worked great as long as we had a simple project, but the moment we went to a multi-module troubles kicked in because we couldn't assure that test tasks from the different projects were not running in parallel.

After a lot of digging through the sbt source in combination with the ParallelExecution page I found out that I could create a custom tag and tie it to my custom scope with the tag SettingsKey.

Let's say it wasn't a pleasant experience and it would be very useful if this awesome feature could be documented on the website. I'm sure many ppl are running into this in large projects that contain integration tests.