Add License

[austinbrown-okta]

Hello 👋🏼, would it be possible to add a LICENSE file to this repository (maybe using the same as is in the original js implementation)?

The code is extremely useful, but for some vendors it cannot be installed as a plugin (i.e., AWS Postgres), meaning we'd need to fork the code into a custom function, but not having a license makes that problematic.

[decoursin]

It looks like the license is in the sql file: https://github.com/geckoboard/pgulid/blob/master/pgulid.sql#L7C23-L7C37

;)