add comment about why the travis exclude matrix is so large
diff --git a/.travis.yml b/.travis.yml
index c784619..92ee221 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -36,6 +36,10 @@
         use_notice: true
         skip_join: true
 
+# When building an exclude matrix on Travis you must supply
+# the exact variable combinations you want to exclude from
+# your build matrix. There is no (current) way to make this
+# less verbose.
 matrix:
     exclude:
         - os: osx