Document "nested workspaces" feature (#491)
Document "nested workspaces" feature (#491)

* Document "nested workspaces" feature

Fixes https://github.com/bazelbuild/continuous-integration/issues/482


bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration