#3: update documentation because the filter cannot apply to the whole CAS server for version < 4

Yes, this filter is to be installed in all CAS servers for version < 4.1 (and I'm going to push the filter by default in CAS server 4.1).

It targets CAS server 4.0, CAS server 3.5.x, etc.

Though, regarding the configuration, there are two options depdending on the version of the CAS server:
- the short mapping for CAS server 4
- the verbose mapping for CAS servers < 4