document the installation prefix `coatjava/`

[c-dilks]

The installation prefix is coatjava/, within the top-level of the source-code directory. Scripts in bin/ are installed (copied) to coatjava/bin/, and those installed scripts are the ones that are supposed to be executed (otherwise "installation directory does not exist" errors will occur if you try to run the scripts directly from bin/).

We should document this stuff, in particular how to execute the scripts. Or figure out a better way...

See https://clas12.discourse.group/t/issues-using-anything-in-bin-after-building-cj/941