repo/main: add module docstrings

This should help people get some bearings in the codebase.

Change-Id: I951238fe617a3ecb04a47ead3809ec72c8fbf5a1
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/231232
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>

main.py

@@ -15,6 +15,12 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+"""The repo tool.
+
+People shouldn't run this directly; instead, they should use the `repo` wrapper
+which takes care of execing this entry point.
+"""
+
 from __future__ import print_function
 import getpass
 import imp