Release Notes: update SPONSORS list

Release Notes: update SPONSORS list

The SPONSORS file is supposed to:

* list Sponsors who paid for significant developments in the squid code,
  donated hardware for devleopment etc.
* exclude cash donations to the project (only cash to a developer for
  specific work)
* in an ordered list broken down in reverse-chronological order of the
  latest release they sponsored anything.

For simplicity of maintenance I'm adding @Squid-X.Y labels at the
boundaries for the bzr copy. They should be stripped out automatically
by the "make dist" process for release packaging.