Update README to mention modules and Go versions. (#15109)
Update README to mention modules and Go versions. (#15109)

@jrmccluskey noticed that the current instructions assume the `go test ./...` work at beam root. This hasn't been true since the katas were added. Instructions have been added to make the current Go version support for development clearer.
robertwb/incubator-beam,lukecwik/incubator-beam,robertwb/incubator-beam,chamikaramj/beam,lukecwik/incubator-beam,robertwb/incubator-beam,chamikaramj/beam,robertwb/incubator-beam,apache/beam,chamikaramj/beam,chamikaramj/beam,chamikaramj/beam,robertwb/incubator-beam,robertwb/incubator-beam,chamikaramj/beam,lukecwik/incubator-beam,robertwb/incubator-beam,apache/beam,lukecwik/incubator-beam,apache/beam,apache/beam,lukecwik/incubator-beam,chamikaramj/beam,lukecwik/incubator-beam,robertwb/incubator-beam,lukecwik/incubator-beam,robertwb/incubator-beam,apache/beam,robertwb/incubator-beam,apache/beam,lukecwik/incubator-beam,lukecwik/incubator-beam,chamikaramj/beam,apache/beam,apache/beam,chamikaramj/beam,apache/beam,chamikaramj/beam,apache/beam,lukecwik/incubator-beam,apache/beam