installation: README.rst with build status badges

• Adds new documentation file README.rst which is essentially identical to README, plus contains extra links to Travis build status badges, bringing them onto the GitHub home page.
  
  This is done because apparently GitHub only recognises file formats by extensions, so badges cannot be easily introduced into plain README files (that we still keep as per GNU autotools practices).

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>