Add overview of functionality/performance for different graph transfo…

[mateoatr]

…rmations in doc

See issue https://github.com/snowleopard/alga/issues/12

[snowleopard]

@mateoatr Many thanks for the PR! I've added a couple of comments above.

Also two general comments:

• For some reason you do not cover the most important data type Graph -- please add it to the tables.
• Could you please make sure the lines fit into around 80 characters, so it'll be easier to check diffs in future?

I haven't yet checked the complexity bounds themselves -- will do that soon.