Added information relevant to the github transition in RELEASE_NOTES.md,
transition document.

Remoted plain text release_notes moving forward.  Created a markdown
version of the README file as an experiment to see what we think. 
GitHub will render markdown natively, as does Doxygen and our new
build system.





git-svn-id: https://sub.unidata.ucar.edu/netcdf/trunk@3376 2e3060e4-1ed8-cf1f-9cc2-9586e844a556