Error Message Related to shared_preload_libraries

We had created an issue #363 about the error message when citus is tried to be loaded by any means than shared_preload_libraries, which there was a PR #436 and it is merged.

The fix involved mentioning putting "shared_preload_libraries" in the postgresql.conf. @anarazel suggested to make this message more informative:

> I don't think mentioning postgresql.conf is a particularly good idea. It can also be a different config file or on the command line. The former is actually a pretty relevant case, e.g. when doing ALTER SYSTEM SET shared_preload_libraries = 'citus';