Added markdown version of RELEASE_NOTES for evaluation.

Modified Doxyfile templates to generate html from the
markdown file (assumes doxygen version 1.8+)

Added link to release notes to mainpage.doc



git-svn-id: https://sub.unidata.ucar.edu/netcdf/trunk@3217 2e3060e4-1ed8-cf1f-9cc2-9586e844a556