Document change to a stable bes user UID and GID

[ndp-opendap]

Prior to recent changes the BES user (aka bes or bes_user) received its UID and GID as part of an RPM install. This meant that the value of the UID and GID had to be determined, later, by interrogation the running docker images.

Why does this matter?

Because when mounting a data volume onto the Hyrax the permissions need to allow for reading (at minimum).

Now these UID and GID values are fixed to:

• bes_user_uid: 101
• bes_user_gid: 1000

Simplifying permission granting on the docker host files system.

Document this in the README.md