edit pr template to include new release process
edit pr template to include new release process

Mentioned in the pr template that the title will be used to generate the
release note and also an optional header section is used to add a
description to the release note.

[#5768]

Signed-off-by: Clara Fu <d5c8bdbc15689555c92563892e959c79b3fc425f@pivotal.io>
Co-authored-by: Taylor Silva <33b075afcd0610c62dcb82fe76b4c1adc69221da@pivotal.io>

concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse