CBMC: Add a link to list of undefined behaviors caught by CBMC

This commit adds a link to the list of undefined behaviors caught
by CBMC to the CBMC README.

Unfortunately, the HTML page targetted by the link is not rendered,
limiting its usefulness. The CBMC team is aware, and we can update
the link once there is a rendered version.

Signed-off-by: Hanno Becker <beckphan@amazon.co.uk>

Author: hanno-becker

proofs/cbmc/README.md

@@ -62,3 +62,5 @@ tests cbmc --help
 ## What is covered?
 
 Each proved function has an eponymous sub-directory of its own. Use [list_proofs.sh](list_proofs.sh) to see the list of functions covered.
+
+The classes of undefined behavior covered by CBMC are documented [here](https://github.com/diffblue/cbmc/blob/develop/doc/C/c11-undefined-behavior.html).