docs: check external repos with taginfo

Add a step to check your work when setting up the new external
repositories, along with some expected output.

docs/source/external_repo_server_bootstrap.rst

@@ -42,6 +42,25 @@ help with these tasks, see the :doc:`server_howto`.
 
     $ koji add-target dist-foo dist-foo-build
 
+  At this point you can verify that your external repository is set with the
+  "taginfo" command. You should see it listed under "External repos". Here is
+  an example with several CentOS external repos::
+
+    $ koji taginfo dist-foo-build
+      Tag: dist-foo-build [740]
+      Arches: x86_64
+      Groups: build, srpm-build
+      Tag options:
+      This tag is a buildroot for one or more targets
+      Current repo: repo#55077: 2017-11-29 03:34:18.847127
+      Targets that build from this tag:
+        dist-foo
+      External repos:
+          2 centos7-cr (http://mirror.centos.org/centos/7/cr/$arch/)
+          3 centos7-extras (http://mirror.centos.org/centos/7/extras/$arch/)
+          5 centos7-updates (http://mirror.centos.org/centos/7/updates/$arch/)
+         10 centos7-os (http://mirror.centos.org/centos/7/os/$arch/)
+
 * Create a ''build'' and ''srpm-build'' group associated with your build tag. ::
 
     $ koji add-group dist-foo-build build