Mention dumping preprocessed input headers in the issue template

Author: fitzgen

.github/ISSUE_TEMPLATE.md

@@ -7,7 +7,11 @@
 // files, and therefore any `#include` harms reproducibility. Additionally,
 // the test case isn't minimal since the included file almost assuredly
 // contains things that aren't necessary to reproduce the bug, and makes
-// tracking it down much more difficult. If necessary, see
+// tracking it down much more difficult.
+//
+// Use the `--dump-preprocessed-input` flag or the
+// `bindgen::Builder::dump_preprocessed_input` method to make your test case
+// standalone and without `#include`s, and then use C-Reduce to minimize it:
 // https://github.com/servo/rust-bindgen/blob/master/CONTRIBUTING.md#using-creduce-to-minimize-test-cases
 ```