Allow debug build using 'make' and document it
Add /p:Configuration=Release to README notes

See https://github.com/fsharp/fsharp/issues/104. Make all builds
produce 'release' if you follow the instructions in the README
Simplify notes on making debug binaries
Merge pull request #110 from funnelweb/fix107

Document how to make 'debug' and 'release' builds on Mac and Windows respectively