Update readme.md to illustrate how to build individual profiles

The following no longer works:

./build.sh clean -P core

giving:

WARNING] The requested profile "core" could not be activated because it does not exist.

This is fine because I recall a discussion on the forums recently where this change was discussed. However, I thought that there would still be an equivalent way of doing this, but looking at the latest readme does not indicate how.

What I'm looking for are the equivalents of the old build profiles.