Add a comment warning developers to push pv tags

The following comment has been added to versions.txt:

While the goal is to be consistent with a SHA on ParaView's main fork, CMB
frequently uses a custom fork of ParaView in order to maintain functionality
while updating the connections between the two projects. When this happens, it
is critical that the forked ParaView also push ParaView's tags to the fork,
since this is how ParaView determines its version (and, resultantly, how it
names its libraries).

To push ParaView's tags:
  $ cd <path-to-sb-build-tree>/superbuild/paraview/src
  $ git remote add kitware https://gitlab.kitware.com/paraview/paraview.git
  $ git fetch --tags kitware
  $ git push --tags origin
Status Job ID Name Coverage
  External
passed ghostflow-branch-check-master