Document non-standard encoding of the PKI token.
Document non-standard encoding of the PKI token.

More details by the code author in his blog post at
http://adam.younglogic.com/2014/02/compressed-tokens/.

Change-Id: I35c5eca2e04a74236bd8c7fb6daab3ea46b59b0e
Closes-Bug: #1352314

sdpp/python-keystoneclient,Mercador/python-keystoneclient,klmitch/python-keystoneclient,sdpp/python-keystoneclient,klmitch/python-keystoneclient,magic0704/python-keystoneclient,Mercador/python-keystoneclient,magic0704/python-keystoneclient