Add links to documentation to readme

[droundy]

I had to do a search on crates.io to identify the crates that comprise this project.

[pcwalton]

There isn't really much documentation yet to link to, unfortunately. What do you mean exactly? The documentation mentions that crates are on crates.io under the pathfinder_ prefix.

[droundy]

Well, identifying those crates by name would be a start. I presume there are docs on docs.rs that could also be linked to.