Merge pull request #2843 from OlegHahm/pkg_porting_patch_readme

pkg: added information about creating a patch

pkg/PORTING.md

@@ -3,9 +3,20 @@ use with RIOT (to build an external module). If you'd like to add a package to
 RIOT you need to add a directory with the name of your package to this directory.
 Your directory should contain at least two files:
 
- * **patch.txt**- Your patch of the upstream application of the package to make
- it build with RIOT.
- * **Makefile**- A Makefile describing how to get the upstream application, apply
- 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
+ * **One or more patch files** - Your patches of the upstream application of
+   the package to make it build with RIOT.
+ * **Makefile**- A Makefile describing how to get the upstream application,
+   apply 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