event: Add documentation

Add documentation for events, including the event command.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/develop/index.rst b/doc/develop/index.rst
index 93ebfa4..2e6d6c3 100644
--- a/doc/develop/index.rst
+++ b/doc/develop/index.rst
@@ -17,6 +17,7 @@
    distro
    driver-model/index
    environment
+   event
    global_data
    logging
    makefiles