Update docs to use new Slack invite website

[Monte9]

We have a public slack reactnativetraining.slack.com. Users would be able to request an invite through the website: https://reactnativetraining.herokuapp.com/

That website was outdated and so I have deployed a new website to serve the same purpose. Now users can join our slack through this website: https://react-native-elements-slack.herokuapp.com/

TODO: We need to update our docs to point to the new website.

[iRoachie]

Where do we have this link right now?

Nevermind, got it

[grasmash]

Looks like https://react-native-elements-slack.herokuapp.com/ is down. Is this still an active community?

[arpitBhalla]

Hey, please join discord, link is available in Readme