Add section on how to build debug build

There is very little documentation on how to build a debug build and a quick google is less than helpful, this makes generating useful core dumps for reported issues harder than it should be.

The idea is that the nice people that help debug issue could have something to point for the common folks that were unlucky enough to hit a segmentation fault but pretty much have been happy using the official binaries and don't know too much about how the build or debug process works.

For extra points, it would be nice if there were an official debug build that one could just download.