Update install documentation
Update install documentation

Details:
  - Added quickstart example to README.rst.
  - Fixed link to Hardware Management Console Web Services API.
  - Updated overview section.

Signed-off-by: Juergen Leopold <2ef4e6fc356c4a0c570adb4e83b64b658f380892@de.ibm.com>

zhmcclient/python-zhmcclient,zhmcclient/python-zhmcclient