[TASK] Copy information from the Pelago site into the Readme.

There is some information on Emogrifier at
http://www.pelagodesign.com/sidecar/emogrifier/ .

These information should also be available in the README.md.
[TASK] Document the usage.
Merge pull request #19 from oliverklee/task/usage

[TASK] Enhance the README.md.