Add some documentation

Change-Id: Ic03d8825c07f948b4069c841232d7f6bff29b874
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..f425dbf
--- /dev/null
+++ b/README.md
@@ -0,0 +1,59 @@
+SUMMARY
+=======
+
+The SuperManifest plugin is for projects that use the 'repo' tool to piece
+together a tree of git repositories. The repo tool reads an XML file that
+describes the layout of the tree.
+
+The file is usually called `default.xml`, and the repository is usually called
+`manifest`.
+
+Notable projects that use 'repo' include the Android Open Source Project.
+
+The plugin will update a superproject to contain the submodules defined in the
+manifest XML file whenever the manifest repo is changed.
+
+
+CONFIGURATION
+=============
+
+The plugin should be enabled in `gerrit.config`, and then configured with a
+`supermanifest.config` configuration file in the `refs/meta/config` branch. The
+`supermanifest.config` lists the mappings from source, eg.
+
+```
+    [superproject "codesearch:refs/heads/stable"]
+      srcRepo = manifest
+      srcPath = default.xml
+      srcBranch = stable
+```
+
+If all branches from the manifest repo should be mirrored into the
+`codesearch` repository, you can specify `*` as the destination
+branch, e.g.,
+
+```
+    [superproject "codesearch:refs/heads/*"]
+      srcRepo = manifest
+      srcPath = default.xml
+```
+
+MANUAL TRIGGER
+==============
+
+Users that have the `administrateServer` permission may simulate an update to
+the manifest repository. This is useful for debugging, and provides diagnostics
+that are otherwise written into the server logs. To do so, issue the following
+call
+
+```sh
+curl -X POST
+  http://HOST/a/projects/codesearch/branches/master/update_manifest
+```
+
+JIRI
+====
+
+There is also support for the Jiri manifest file used in the Fuchsia OS.
+
+TODO(anmittal): provide more documentation.