Finish up README

[esttorhe]

The README was in a «beautification» process when 2.1.1 was pushed to fix a minor issue.

Need to finish explaining what RxViewModel is so potential users won't need to check out another pod's page to know what this one is about.

[AliSoftware]

I like having short examples/extract of code in the README to see what the API looks like at a glance by just reading the README (without even pod try or opening the example project)