add Composer and Packagist to release process

[jeffpaul]

Is your enhancement related to a problem? Please describe.
On most plugins we include a composer.json file, but we don't always submit a package to Packagist. Let's update our docs to confirm that as part of the process with specific steps on how to do so.

Describe the solution you'd like

• add composer.json details to Release Process
• add Packagist details to Release Process

Designs
n/a

Describe alternatives you've considered
none

Additional context
n/a