Document how to create a patch file

[zao]

While the documentation mentions what kind of format that patches need to be in there is no information on how one would create a patch to begin with. The question has come up a few times on Slack.

For example, my workflow for creating a patch is something like this, I'm sure there's a lot of variations on it among contributors and maintainers:

• unpack the source tarball twice, once into foo-1.2.3.orig and once into foo-1.2.3
• make the desired changes to foo-1.2.3
• diff -ruN foo-1.2.3.orig foo-1.2.3 > foo-1.2.3_fix-some-problem.patch
• edit the patch file and add a plain text section at the top of the file describing what it solves, where it's sourced from, credits/authors
• add to desired easyconfig together with the computed checksum or suggest --inject-checksums