orangefs: document package install and xfstests procedure

Unless one is working on the userspace code, there's no need to compile
OrangeFS.  The package works just fine.

(But leave the documentation for building from source since not everyone
uses distributions which include the package.)

Also document the process to run xfstests against OrangeFS.

Signed-off-by: Martin Brandenburg <martin@omnibond.com>
Signed-off-by: Mike Marshall <hubcap@omnibond.com>