HACKING: add guidelines to patch submission

HACKING

@@ -91,3 +91,22 @@ Other debugging settings that can be toggled:
 
  - Environment variable OFONO_AT_DEBUG (set to 1): enable AT commands
    debugging
+
+
+Submitting patches
+==================
+
+If you fixed a bug or you want to add support for something, patches are
+welcome! In order to ease the inclusion of your patch, it's important to follow
+some rules, otherwise it will likely be rejected by maintainers:
+
+1) Do *not* add "Signed-off-by" lines in your commit messages. oFono does not
+use them, so including them is actually an error.
+
+2) Be sure to follow the coding style rules of oFono. They are listed in
+doc/coding-style.txt.
+
+3) Split your patch according to the top-level directories. E.g.: if you added
+a feature that touches files under 'include/', 'src/' and 'drivers/'
+directories, split in three separated patches, taking care not to
+break compilation.