README: add note on patching

README.md

@@ -159,6 +159,21 @@ cd $GENODE_DIR
 make -C build/x86_64 run/bomb
 ```
 
+## Format a patch to the Genode sources
+
+```shell
+git format-patch genodelabs/master --stdout > ../genodepkgs/packages/genodelabs/foo.patch
+```
+
+[targets.nix](./packages/genodelabs/targets.nix)
+```nix
+{
+  …
+  foo.patches = [ ./foo.patch ];
+  …
+}
+```
+
 # Glossary
 
 Some of terms that follow are necessary for understanding packaging methodology,