Add deprecation notice

[drigz]

This project is no longer maintained by Google (and has not been for some time) so will be moved to the new "ng-log" org.

Before merging this, we must:

• cut a new release including the rename to ng-log
• fix the documentation link at https://github.com/ng-log/ng-log
• ensure ng-log shows up at https://registry.bazel.build/search?q=ng-log
• fix Bazel build instructions work at https://github.com/ng-log/ng-log/blob/master/docs/build.md

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.22%. Comparing base (6c5c692) to head (3809ada).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1150   +/-   ##
=======================================
  Coverage   62.22%   62.22%           
=======================================
  Files          19       19           
  Lines        2324     2324           
  Branches      845      845           
=======================================
  Hits         1446     1446           
  Misses        596      596           
  Partials      282      282