lambda: set slack \@handle

[maxenglander]

It's nice to be able to set @- handle in Slack topic because it allows person to be notified by Slack when they go oncall, and because other people can click that handle to go to DM or get info about the oncall person.

This commit makes this the new behavior in lambda/main.py. Because this behavior requires getting Slack user from the Slack API, and because Bot users don't have that permission, this commit also updates README.md with instructions on creating an App user with sufficient permissions.