Add Squot and Metacello section to README

[LeonMatthes]

Due to the way Squot and Metacello interact with one another, it is necessary for Metacello-projects that use Squot to add a certain piece of code to their baseline.

Add instructions for this to the README

[LinqLover]

Hey @j4yk, would this be a reason to provide a subclass of BaselineOf for every Squot project baseline? Just asking, my knowledge about the Metacello/FileTree architecture is still very limited, but I just stumbled upon Metacello/metacello#513 for the third or fourth time and think that the default process for setting up a baseline for Squot project is still too sophisticated. What do you think? :-)

[j4yk]

I don't think that Squot should provide an abstract baseline class for projects. I could imagine a "Create baseline" feature in the project menu though. People may expect more of this than just an empty baseline subclass with the correct projectClass method, though.