event: Add documentation

Add documentation for events, including the event command.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 0aacf53..7501028 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -29,6 +29,7 @@
    x86/cbsysinfo
    conitrace
    echo
+   event
    exception
    extension
    exit