Improve documentation

Add `Advantages of wheels` and `Other Resources` sections. Split information about creating wheels into `Pure Python` and `C extensions` subsections. Add link to pull-requests on github to encourage more active participation.

I have tried to cover the suggestions made in [issue #29](/meshy/pythonwheels/issues/29#issuecomment-35823104) by @zzzeek. I have also included suggestions from #22 and #16.