pkg: added information about creating a patch

pkg/PORTING.md

@@ -9,3 +9,14 @@ Your directory should contain at least two files:
  the patch and how to build the package as a RIOT module.
  A rough template for several methods of acquiring a package
  is provided in Makefile.git, Makefile.http, and Makefile.svn
+
+### Creating a patch with git
+
+Assuming your upstream application resides in a git repository, you can create
+the patch files as follows:
+ * checkout the targeted version of the upstream application
+ * conduct necessary changes (e.g. edit, add, or remove some files)
+ * commit your changes using `git commit`
+ * create the patch files using `git format-patch -n HEAD~N` where `N` is the
+   number of commits you did
+ * move the resulting patch files to the corresponding subfolder of pkg