Readme with provider table

I converted the list of providers into a table and split address- and ip-based geocoders (datasciencetoolkit is in both tables). I also added most terms and limit. Actually the terms of service of some providers are quite strict. Google and Bing for example allow geocoding only to be used with their maps, others require strict attribution or forbid storing or caching of any kind...

In my opinion it's more read-able but I can understand if you disagree or prefer text-only lists.

Disclaimer: I work for OpenCage so I had a ready list of terms from various providers in an Excel spreadsheet already.