Add a license to improve discoverability

[klaude]

Howdy, @kraney. One of our team members noticed that we can't browse the contents of this library on pkg.go.dev. For instance, the accounts_and_users package won't display due to pkg.go.dev's license policy. If you adopt one of their supported licenses then others will be able to see README contents and other package information, which should lead to more adoption of the library. Just thought we'd point it out. Thanks for making this!

[kraney]

An oversight, thanks for pointing it out!