docs: Add info to README for allowed markup
docs: Add info to README for allowed markup

Add explanation of how markup is allowed
and how to change it in 'extending' section
of README.

tstillwell2k17/GAE-BLOGAPP-MULTI_USER-DEMO,tstillwell2k17/GAE-BLOGAPP-MULTI_USER-DEMO,tstillwell2k17/GAE-BLOGAPP-MULTI_USER-DEMO