Docs: Update to download sources manually, not through apt source

[danirabbit]

What Happened

The currently documented method is to use apt-source to download the package to be updated. However, it appears there can be issues here with patches which are already applied to the sources fetched via apt. In the build process on launchpad, quilt with throw errors about patches which can be "reverse applied".

Expected Behavior

It seems like if we get the sources by manually downloading them from packages.ubuntu.com that we don't get these build errors. So we need to update the README docs to recommend getting the sources this way