Update ISSUE_TEMPLATE.md

.github/ISSUE_TEMPLATE.md

@@ -1,3 +1,5 @@
+([x] becomes a filled in checkbox, [ ] is an empty one)
+
 - [ ] I have verified there are [no duplicate active or recent bugs, questions, or requests](https://github.com/afollestad/material-dialogs/issues?q=is%3Aissue+is%3Aclosed)
 - [ ] I have verified that I am using the latest version of Material Dialogs.
 - [ ] I have given my issue a non-generic title.