crashes: add README

Author: matthiaskrgr

tests/crashes/README.md

@@ -0,0 +1,13 @@
+This is serves as a collection of crashes so that accidental ICE fixes are tracked.
+This was formally done at https://github.com/rust-lang/glacier but doing it inside
+the rustc testsuite is more convenient.
+
+It is imperative that a test in the suite causes an internal compiler error/panic
+or makes rustc crash in some other way.
+Accepted exit codes are: 101 (may be expanded later)
+
+When adding crashes from https://github.com/rust-lang/rust/issues, the
+issue number should be noted in the file name (12345.rs should suffice).
+
+If you happen to fix one of the crashes, please move it to `tests/ui`! :) 
+