Add mention-bot config (#5788)

- Limit notifications to members of the Mapbox GitHub organization.
- Tweak the message a bit for concision and punctuation.
- Experimentally bump number of PR files checked from 5 to 10.

.mention-bot

@@ -0,0 +1,5 @@
+{
+  "requiredOrgs": ["mapbox"], // mention-bot will only mention user who are members of one of these organizations.
+  "message": "@pullRequester, thanks for your PR! By analyzing this pull request, we identified @reviewers to be potential reviewers.",
+  "numFilesToCheck": 10, // Number of files to check against, default is 5.
+}