README.source: Update "Updating the upstream source" to reflect current practice

* Recommend using git rather than tarballs
* We compress upstream source with xz, not gzip
* Don't mention the complicated alternative to debian/rules orig

debian/README.source

@@ -1,24 +1,25 @@
 Updating the upstream source
 ============================
 
-1) You can use either:
-   a) a git repository of the kernel source
-   b) a kernel tarball from kernel.org (e.g. linux-3.4.tar.bz2)
-      and, optionally, a patch (e.g. patch-3.5-rc1.bz2).
+In addition to the build-dependencies, you will need the rsync and
+unifdef packages installed.
 
-2) Run ./debian/bin/genorig.py <repository>
-   or  ./debian/bin/genorig.py <tarball> [patch]
-   This will produce ../orig/linux_<version>.orig.tar.gz
-   (e.g. linux_3.5~rc1.orig.tar.gz).
+1) It is recommended to fetch the release tag from the relevant upstream git
+   repository, one of:
 
-   (genorig.py requires the python and unifdef packages to be
-    installed)
+   * https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
+   * https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git
+   * git://kernel.ubuntu.com/ubuntu/linux.git
 
-3) Unpack linux_<version>.orig.tar.gz, cd into the new directory,
-   and do a 'git archive' to get the debian/ subdirectory.
-   Alternatively unpack using "make -f debian/rules orig".
+   However, it is also possible to use upstream tarball and patch releases.
 
-   (the orig target of the Makefiles requires rsync)
+2) Run: ./debian/bin/genorig.py <repository>
+   or:  ./debian/bin/genorig.py <tarball> [patch]
+
+   This will produce ../orig/linux_<version>.orig.tar.xz
+   (e.g. linux_3.5~rc1.orig.tar.xz).
+
+3) Run: make -f debian/rules orig
 
 Applying patches to the Debian kernel tree
 ==========================================