Add link to rules_pkg docs page.
This will make it a little easier to find the replacement text for this page.
Closes
RELNOTES: None
## Deprecated
These rules have been extracted from the Bazel sources and are now available at
-[bazelbuild/rules_pkg](https://github.com/bazelbuild/rules_pkg/releases).
+[bazelbuild/rules_pkg](https://github.com/bazelbuild/rules_pkg/releases)
+ [(docs)](https://github.com/bazelbuild/rules_pkg/tree/master/pkg).
Issues and PRs against the built-in versions of these rules will no longer be
addressed. This page will exist for reference until the code is removed from
