show how to view docs locally

Author: tshepang

tests/crashes/README.md

@@ -1 +1,9 @@
-Documentation on crash tests is at https://rustc-dev-guide.rust-lang.org/tests/compiletest#crash-tests
+Documentation on crash tests is at https://rustc-dev-guide.rust-lang.org/tests/compiletest#crash-tests.
+
+Alternatively, you can build the documentation from this repository:
+
+```console
+mdbook serve --open src/doc/rustc-dev-guide
+```
+
+The documentation will then be available at http://localhost:3000/tests/compiletest.html#crash-tests.