Add an issue template explaining the new/old book business

.github/ISSUE_TEMPLATE.md

@@ -0,0 +1,23 @@
+## What to expect when you file an issue here
+
+This is the repo for an in-progress rework of The Rust Programming Language book
+available at http://rust-lang.github.io/book/.
+
+If you are filing an issue about the existing book available at
+https://doc.rust-lang.org/book/, please note that we are now concentrating on
+the new version of the book.
+
+We would still love feedback on the existing book as well as the new book, just
+know that one of a few things may happen with your issue:
+
+- If we have not gotten to that topic in the new book, we may leave a note for
+  ourselves about this issue to address when we get there.
+- We may have already addressed this issue in a rewritten version of the topic
+  relating to this issue, so we might close it.
+- If this issue still applies to the new book, we will fix it.
+
+If you have time before filing this issue, please take a look at the new book
+to see if this issue has been addressed and include that information in the
+issue.
+
+Thank you for reading, you may now delete this text!