Expand on our existing slack guidelines for maintainers and using `@(here|channel)` messaging

[majormoses]

Context: in slack we restrict @(here|channel) messages to only be sent by admins. We do this to protect our users from a new user to slack who does not realize that there are multiple people across multiple time zones. An unfortunate side effect of this that a user can not mute such notifications and means that they can only either mute the whole channel (which I believe would suppress such notifications) or leave the channel if they do not wish to receive notifications for these. We generally only make such notifications in #announcements channel which can only be sent by admins and have historically been very low volume.

As our community engagement grows on slack we must be more vigilant in our usage and learn from our mistakes. After a very good and lengthy discussion in slack about @(here|channel) notifications here are the action items:

• Create some guidelines for new maintainers for slack to help guide them on when they should and should not use admin powers to notify large groups of individuals
• Ensure that on-boarding process includes reading through the guidelines before they are given access

[majormoses]

This should most likely be documented here: https://github.com/sensu-plugins/community/blob/master/HOW_WE_SLACK.md