Add versioning

[lestofante]

Issue description

I am packaging processing for distribution on Arch Linux AUR, problem is as there is no official versioning, not there or on the main website, and there is no easy way to

• verify checksum without random breakage
• checkout a specific version
• use existing tool to check for new release

URL(s) of affected page(s)

this repo and https://download.processing.org/reference.zip

Proposed fix

add git tag for versioning or eqivalent

[REAS]

Please add this to https://github.com/processing/processing/issues, that's where software releases are made.