Document debug vs. release builds

[johnomotani]

The 'getting started' section of the documentation currently the configurations we suggest would end up with builds with a fair amount of debugging enabled - CHECK=2 for autotools or -DCMAKE_RELEASE_TYPE=Debug for CMake. This will probably lead to slow builds. We recommend optimised build in the machine-specific section, but a discussion of debug vs. release in the getting started section would probably be helpful for new users.