doc: global data pointer

Add the description of the global data pointer to the generated HTML
documentation.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/develop/index.rst b/doc/develop/index.rst
index 98a95ad..89e80ea 100644
--- a/doc/develop/index.rst
+++ b/doc/develop/index.rst
@@ -9,4 +9,5 @@
 
    coccinelle
    crash_dumps
+   global_data
    logging