Add comment line for initialization of the Slack object.

[Helveg]

The README states:

$Slack = new Slack('xoxp-0123456789-0123456789-0123456789-1a1a11');

But I don't know what that value is, is that a token of some sort? Could you add a comment line specifying what value to fill in there?

[yongzhenlow]

@Helveg In earlier implementations that was how authorizations worked, you can read more here https://api.slack.com/custom-integrations/legacy-tokens. The readme could use some updates, thanks for sharing!