Add License

[ianstanton]

Hey @viggy28! Thanks for building the pg_savior extension :) I noticed there was no LICENSE file. What license does this project have?

[theory]

Should also be applied to the META.json file in #2.

[viggy28]

Welcome @ianstanton. Added licensing info.

Also, created an account in PGXN. Will do the rest of the steps once it's approved.