README: improve PAM configuration documentation (again)

README: improve PAM configuration documentation (again)

Make some more corrections:

- pam-config-framework isn't actually Ubuntu-specific but actually
  applies to Debian and any Debian derivative.

- The pam-config-framework file is indeed installed by `make install`,
  just not into the correct location.

- On Debian (and Debian derivatives), the PAM configuration isn't
  actually part of the 'fscrypt' package but rather 'libpam-fscrypt'.

- Clarify where to add the pam_fscrypt.so session hook.