Add License file for the Go SDK  for pkg.go.dev
Add License file for the Go SDK  for pkg.go.dev

This permits the SDK content and document to appear on https://pkg.go.dev/ per https://pkg.go.dev/license-policy

Content for the file was automatically provided by github's license tool.
apache/beam,lukecwik/incubator-beam,apache/beam,robertwb/incubator-beam,chamikaramj/beam,lukecwik/incubator-beam,chamikaramj/beam,lukecwik/incubator-beam,robertwb/incubator-beam,chamikaramj/beam,lukecwik/incubator-beam,apache/beam,apache/beam,robertwb/incubator-beam,apache/beam,lukecwik/incubator-beam,apache/beam,iemejia/incubator-beam,chamikaramj/beam,robertwb/incubator-beam,iemejia/incubator-beam,apache/beam,lukecwik/incubator-beam,robertwb/incubator-beam,lukecwik/incubator-beam,chamikaramj/beam,robertwb/incubator-beam,lukecwik/incubator-beam,lukecwik/incubator-beam,robertwb/incubator-beam,apache/beam,apache/beam,chamikaramj/beam,robertwb/incubator-beam,robertwb/incubator-beam,apache/beam,chamikaramj/beam,chamikaramj/beam,apache/beam,chamikaramj/beam,robertwb/incubator-beam,chamikaramj/beam,lukecwik/incubator-beam