Building commands improvement

[kmturley]

Currently BUILDING.md contains the commands:

cmake -G "Unix Makefiles" .
make

However this then mixes build files with source files. It's generally best practice to create a build folder:

cmake \
  -G "Unix Makefiles" \
  -S . \
  -B ./build
make --directory ./build

Then all build files are kept nicely in one folder. You could also add the run command:

./build/main/mrswatson64 --help