crash test readme: point to rustc-dev-guide

There is information that is duplicated here and in rustc-dev-guide, and the latter is more comprehensive, so point there.

Author: tshepang

tests/crashes/README.md

@@ -1,25 +1 @@
-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.
-A test will "pass" if rustc exits with something other than 1 or 0.
-
-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)
-and also inside the file via `//@ known-bug #4321` if possible.
-
-If you happen to fix one of the crashes, please move it to a fitting
-subdirectory in `tests/ui` and give it a meaningful name.
-Also please add a doc comment at the top of the file explaining why
-this test exists. :)
-Adding
-Fixes #NNNNN
-Fixes #MMMMM
-to the description of your pull request will ensure the
-corresponding tickets will be closed automatically upon merge.
-The ticket ids can be found in the file name or the `known-bug` annotation
-inside the testfile.
-
-Please do not re-report any crashes that you find here!
+Documentation on crash tests is at https://rustc-dev-guide.rust-lang.org/tests/compiletest#crash-tests