Document how to work with private git repositories

Description of the problem / feature request:

It's unclear how to use a private git repository with bazel. Please consider documenting how to set this up more prominently in the repository documentation.

Feature requests: what underlying problem are you trying to solve with this feature?

Interactive commands tend to fail because the git + ssh handshake may need the user to enter a password to unlock the ssh private key.

What operating system are you running Bazel on?

Ubuntu 20.04

What's the output of bazel info release?

release 3.2.0

Have you found anything relevant by searching the web?

• jayconrad's article: https://jayconrod.com/posts/114/accessing-private-github-repositories-over-http
• rules_go issue: https://github.com/bazelbuild/rules_go/issues/1280
• google groups discussion: https://groups.google.com/forum/#!topic/bazel-discuss/geO8pgwxMBY
• rules_jvm_external issue related to private repo auth: https://github.com/bazelbuild/rules_jvm_external/issues/80

Example error message

ERROR: /home/red/git/blah/BUILD.bazel:3:11: //cmd/downloader:go_default_library depends on @private_thing//:go_default_library in repository @private_thing which failed to fetch. no such package '@private_thing//': failed to fetch private_thing: # cd .; git clone git@github.com:gonzojive/blah.git /home/red/.cache/bazel/_bazel_red/11b28b8a9f8bcb1f153ff86ce3120090/external/private_thing