binman: doc: Add documentation to htmldocs

Add a link to binman's documentation and adjust the files so that it is
accessible. Use the name README.rst so it is easy to discover when binman
is installed without U-Boot.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/develop/index.rst b/doc/develop/index.rst
index 444df67..3edffbc 100644
--- a/doc/develop/index.rst
+++ b/doc/develop/index.rst
@@ -26,6 +26,14 @@
    crash_dumps
    trace
 
+Packaging
+---------
+
+.. toctree::
+   :maxdepth: 1
+
+   package/index
+
 Testing
 -------