Update README with a bit more info

The information in README.rst is used to describe the python-ironicclient
package at pypi.python.org: https://pypi.python.org/pypi/python-ironicclient.

It doesn't have very much information right now. This adds a bit more
information; in particular, a link to the dev documentation.

Change-Id: Idb28b95b32617a325949c5f694d40b3834069d3f
Partial-Bug: #1314873
Merge "Update README with a bit more info"