Add link to Slack and minor  grammatical edits. (#5723)
Add link to Slack and minor  grammatical edits. (#5723)

Add link to Slack and minor grammatical edits.

Summary:
Added a link to the Mesos Slack.

JIRA issues:
mesosphere/marathon,gsantovena/marathon,guenter/marathon,guenter/marathon,mesosphere/marathon,meln1k/marathon,gsantovena/marathon,gsantovena/marathon,gsantovena/marathon,janisz/marathon,guenter/marathon,guenter/marathon,guenter/marathon,meln1k/marathon,mesosphere/marathon,gsantovena/marathon,janisz/marathon,meln1k/marathon,janisz/marathon,janisz/marathon,meln1k/marathon,meln1k/marathon,mesosphere/marathon,mesosphere/marathon,janisz/marathon