expo: Add documentation

Add some documentation for the expo feature.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/develop/index.rst b/doc/develop/index.rst
index 97c526e..79d7736 100644
--- a/doc/develop/index.rst
+++ b/doc/develop/index.rst
@@ -34,6 +34,7 @@
    distro
    driver-model/index
    environment
+   expo
    event
    global_data
    logging