doc: describe our documentation style

Provide a reference document for the U-Boot documentation style.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/develop/index.rst b/doc/develop/index.rst
index a52ad63..ddbf8da 100644
--- a/doc/develop/index.rst
+++ b/doc/develop/index.rst
@@ -11,6 +11,7 @@
 
    codingstyle
    designprinciples
+   docstyle
    patman
    process
    release_cycle