chore(README): add the travis badge lower

the travis badge was located above the title which looked a bit weird,
move it down.
chore(README): add an introduction section to the Overview

Add an introduction to the overview and put a link at the bottom.
feat(README): reorganize assuming someone knows raft

People who find this repo probably already know a bit about raft. So,
lets tell them about the current state of the project, the features that
are implemented and the example projects first.
Merge pull request #104 from philips/cleanup-README

Cleanup readme