Add the mention-bot configuration file (GH-352)

Discussed at https://github.com/python/core-workflow/issues/32.

.mention-bot

+{
+  "maxReviewers": 5,
+  "numFilesToCheck": 10,
+  "findPotentialReviewers": true,
+  "fileBlacklist": [
+    "Misc/ACKS",
+    "Misc/NEWS"
+  ],
+  "userBlacklist": [],
+  "userBlacklistForPR": [],
+  "requiredOrgs": ["python"],
+}