Add link to Socket Mode example in README.md

[itayd]

Hi, I propose linking to the Socket Mode example from the README.md file. As I understand, Socket Mode is preferable to RTM, and I got confused between the two. See #1015 for suggested modification.

See https://api.slack.com/rtm for note: "For most applications, Socket Mode is a better way to communicate with Slack."

Thanks.