[BEAM-10753] Add Slack link invitation on README
[BEAM-10753] Add Slack link invitation on README

Using Slack for communication will be easier.
We also can find the link invitation on https://cwiki.apache.org/confluence/display/INFRA/Slack+Guest+Invites
However, it's not directly listed on contact us, at least for now.

chamikaramj/beam,lukecwik/incubator-beam,chamikaramj/beam,lukecwik/incubator-beam,chamikaramj/beam,robertwb/incubator-beam,chamikaramj/beam,lukecwik/incubator-beam,robertwb/incubator-beam,lukecwik/incubator-beam,lukecwik/incubator-beam,chamikaramj/beam,robertwb/incubator-beam,apache/beam,apache/beam,robertwb/incubator-beam,lukecwik/incubator-beam,apache/beam,apache/beam,chamikaramj/beam,robertwb/incubator-beam,robertwb/incubator-beam,chamikaramj/beam,apache/beam,lukecwik/incubator-beam,chamikaramj/beam,apache/beam,apache/beam,apache/beam,robertwb/incubator-beam,chamikaramj/beam,robertwb/incubator-beam,lukecwik/incubator-beam,robertwb/incubator-beam,apache/beam,robertwb/incubator-beam,apache/beam,chamikaramj/beam,lukecwik/incubator-beam,apache/beam,lukecwik/incubator-beam