Added 'Documentation' topic with 2 tools

I have been researching documentation platforms for training and knowledge transfer at my current company. I have added Hackpad and GoogleDocs so far.

Might be worth introducing a few new tags, possibly 'web' (or something similar to denote a web platform), this would be useful for describing any SaaS, PaaS and IaaS services. Also 'api' to denote a tool that exposes an API to further integrate with it (both Hackpad and GoogleDocs have one).