Mention use of pristine-tar in README.source.
Mention use of pristine-tar in README.source.

tchx84/debian-pkg-sugar-toolkit,tchx84/debian-pkg-sugar-artwork,tchx84/debian-pkg-sugar-artwork,tchx84/debian-pkg-sugar-toolkit,tchx84/debian-pkg-sugar-base,tchx84/debian-pkg-sugar-artwork,tchx84/debian-pkg-sugar-base,tchx84/debian-pkg-sugar-toolkit,tchx84/debian-pkg-sugar-base