Add triagebot config to aid with code review

Author: senekor

triagebot.toml

@@ -0,0 +1,9 @@
+# Enable comments linking to triagebot range-diff when a PR is rebased
+# onto a different base commit
+# Documentation at: https://forge.rust-lang.org/triagebot/range-diff.html
+[range-diff]
+
+# Adds at the end of a review body a link to view the changes that happened
+# since the review
+# Documentation at: https://forge.rust-lang.org/triagebot/review-changes-since.html
+[review-changes-since]