[op25-dev] Build instructions for new release

Software prerequisites include GNU Radio 3.7; also cmake; as well as the current list of build packages listed on the build instructions page http://op25.osmocom.org/trac/wiki.png/wiki/BuildInstructionsPage Once this is done and the tar.gz file has been downloaded and unpacked from http://mattrobert.com/Wireless/op25-gr3.7-0.01.tar.gzThe two subdirectories, gr-op25 and gr-op25_repeater must be built as follows cd ......./gr-op25 mkdir build cd build cmake ../ make sudo make install [repeat the above commands for gr-op25_repeater] cd ......./gr-op25_repeater mkdir build cd build cmake ../ make sudo make install