warn contributors not to edit the .gitignore for their own stuff

Signed-off-by: Jess Frazelle <acidburn@google.com>

.gitignore

@@ -1,2 +1,6 @@
+# hoard project generated files to ignore
+#  if you want to ignore files created by your editor/tools,
+#  please consider a global .gitignore https://help.github.com/articles/ignoring-files
+#  please do not open a pull request to add something created by your editor or tools
 hoard
 testhoard