README: Update the build instructions

The build instructions had several issues:
    - They referred to a non-existent file, '/doc/building.txt'.
    - They instructed the user to explicitly enable support for every
      desired platform. But Waffle's CMake can now autodetect that.

Reported-by: Emil Velikov <emil.l.velikov@gmail.com>
Merge branch 'origin/master' into next