docs/manual: document the new patch naming convention

To ease generating patches, we now use a naming convention that is in
line with what git-format-patch does, that is:
  - do not prefix patches with the package name
  - prefix patches with a 4-digit mber
  - start numbering at 0001

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>