More textshapes and documentation of them

[aedobbyn]

@cortinah triangle_string and center_string are in the most recent version of the package, but I haven't put anything in the readme about what they do or examples of them. If you have a cool use case for them (could just be the Star Wars intro) we can stick that in the readme. Could also think of how to make some other shapes.

[cortinah]

Thank you for the suggestion. Will put something together in the next few days.

[aedobbyn]

Sounds good :)