luci: minor edits to CONTRIBUTING.md & PR template

luci: minor edits to CONTRIBUTING.md & PR template

For PR template, minor fix to 'please read'
section, as symbol spacing was different
on GitHub website than on GitHub Desktop.
Added HTML comment with example on how
to complete the checklist using [x]. Added
examples to the 'tested on' section.
Added abbreviation explanation for PR to
better understand terminology, similar to
using S.O.B. For CONTRIBUTING.md,
bumped up release branch example, and
minor capitalisation fixes for PR.

Signed-off-by: Joshua Criss <redacted>