Added a mentionbot blacklist in prep for enabling it (#510)

diff --git a/.mention-bot b/.mention-bot
new file mode 100644
index 0000000..9a20eb3
--- /dev/null
+++ b/.mention-bot
@@ -0,0 +1,3 @@
+{
+    "userBlacklist": ["exarkun"]
+}
diff --git a/MANIFEST.in b/MANIFEST.in
index 50cdc78..72b419c 100644
--- a/MANIFEST.in
+++ b/MANIFEST.in
@@ -7,3 +7,4 @@
 recursive-exclude   leakcheck   *.py *.pem
 prune               doc/_build
 prune               .travis
+prune               .mention-bot