Update documentation
Update documentation

The documentation was misleading in two ways:

1) appToken needs to be passed as a named parameter in order to be recognized.
2) the "Send a message" example did not actually start the socketModeClient, which
meant the process would immediately terminate.
slackhq/node-slack-client,slackapi/node-slack-sdk,slackapi/node-slack-sdk,slackapi/node-slack-sdk,slackapi/node-slack-sdk