Extend bridge go-slack documentation for authentication via login-token

[rzerres]

For the go slack-bridge, the given documentation that describes the authentication process is a bit sparse.

Slack has enhanced the access-security to a given domain. The
login-password method will not be handled correctly, since you need
to use a 2FA method.

Using login-token enables a secure and working solution.
Admins are not neccessarily confident to extract the needed token and their corresponding cookies.
The PR will the needed steps precisely when consuming the web-browsers dev-tools.

That should ease the process.
For the go slack-bridge, the given documentation that describes the authentication process is a bit sparse.

Slack has enhanced the access-security to a given domain. The
login-password method will not be handled correctly, since you need
to use a 2FA method.

Using login-token enables a secure and working solution.
Admins are not neccessarily confident to extract the needed token and their corresponding cookies.
The PR will the needed steps precisely when consuming the web-browsers dev-tools.

That text should ease the process.