layers/balena-beaglebone: Readme update
layers/balena-beaglebone: Readme update

Replace the old name 'resin' with 'balena'. It matches now the source
code directory structure. Update pull request guidelines in readme, from
balena-os/balena-intel layer. They were outtdated.

Changelog-entry: Replace 'resin' with 'balena' and update pull request rules.

resin-os/resin-beaglebone,resin-os/resin-beaglebone