add doc.go, update README, and tag a release to make online godocs get refreshed

[fxamacker]

Issue To Be Solved

doc.go is missing and pkg.go.dev doesn't currently show API docs for atree.

Suggested Solution

• add doc.go with Atree package description
• if time allows, also update README with a bit more info about Atree
• tag a release because that helps pkg.go.dev display the updated version