Adds a note on Mattermost smtp configuration keys
Adds a note on Mattermost smtp configuration keys

As noted in #1287 the Mattermost and GitLab configuration keys differ

This commit adds a note to this effect in the Mattermost README.

Fixes #1287

gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab