include issue tracker link in the package and top-level READMEs

[vchudnov-g]

In each directory under packages/, we have a .repo-metadata.json file that includes a field issue_tracker. We should include that link in the generated package-level README.rst, and arguably also in the list of packages included in the top-level README.rst

When this is done, we can give easier instructions (eg on issue templates) on how to get support.