CONTRIBUTING: document what a patch has to look like

CONTRIBUTING: document what a patch has to look like

Three things are expected of a patch under <package>/patches/ and none of
them were written down for contributors.

The header has to be git am compatible, with the From/From:/Date:/Subject:
lines, which the formality check enforces. The body has to say where the
change stands with respect to upstream, so a later maintainer can tell a
backport that a version bump makes redundant from a cross-compile fix that
has to stay. And the diff has to come from the buildroot's refresh target,
which invokes quilt with -p ab --no-index --no-timestamps and
QUILT_DIFF_OPTS=-p; a git diff carries index lines and timestamps and lacks
the function name in the hunk headers, so it gets sent back even when it
applies.

Write all three down, and say what each quilt option is for rather than
just naming the command, so the reason survives the next time someone
wonders whether the timestamps matter.

llm-review-rules.md already tells the review bot about the refresh target
and the upstream reference. That file is the bot's instructions and it
points contributors here, so this is where the requirement belongs.

Signed-off-by: Josef Schlehofer <redacted>