Add blurb on configuration and run_mirror in README

Currently the configuration syntax is only documented in the run_mirror
source. Adding enough information to the RAEDME for users to get up and
running with a simple configuration is much more user friendly.

Change-Id: I54d978ac127dfe37348167f3a2b08cc09119619b

README.rst

@@ -10,3 +10,32 @@ things you expect to download from PyPI.
 
 pypi-mirror will build a local static mirror for you based on requirements
 files in git repos.
+
+
+Configuration
+-------------
+
+A YAML configuration is needed to create a mirror. Below is an example
+configuration. ::
+
+  cache-root: /tmp/cache
+
+  mirrors:
+    - name: openstack
+      projects:
+        - https://git.openstack.org/openstack/requirements
+      output: /tmp/mirror/openstack
+
+    - name: openstack-infra
+      projects:
+        - https://git.openstack.org/openstack-infra/config
+      output: /tmp/mirror/openstack-infra
+
+
+Creating a mirror
+-----------------
+
+The run_mirror utility creates a mirror. ::
+
+  run-mirror -c mirror.yaml
+