[gnome-builder] doc: update greeter doc

[Christian Hergert <chergert src gnome org>]

commit 93e53d630c3bf79d61a59e0a6bdba58afba866b7
Author: Christian Hergert <chergert redhat com>
Date:   Mon Mar 11 21:25:35 2019 -0700

    doc: update greeter doc

 doc/help/plugins/greeter.rst | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/doc/help/plugins/greeter.rst b/doc/help/plugins/greeter.rst
index b50bea371..540ea4690 100644
--- a/doc/help/plugins/greeter.rst
+++ b/doc/help/plugins/greeter.rst
@@ -56,4 +56,5 @@ Adding Project Sections
 
 The list of projects in the greeter are broken into sections.
 You can add a section to the greeter using ``greeter_workspace.add_section()``.
+The section should implement ``Ide.GreeterSection`` so that it can respond to user input and selections.