doc: Document the usage of --enable-undefined-sanitizer

Signed-off-by: Donald Sharp <sharpd@nvidia.com>

doc/developer/workflow.rst

@@ -1306,6 +1306,16 @@ MemorySanitizer
 
    to ``configure``.
 
+UndefinedSanitizer
+   Similar to AddressSanitizer, this tool provides runtime instrumentation for
+   detecting use of undefined behavior in C.  Testing your own code with this
+   tool before submission is encouraged.  You can enable it by passing::
+
+      --enable-undefined-sanitizer
+
+    to ``configure``.  If you run FRR with this you will probably also have
+    to set ``sudo sysctl vm.mmap_rnd_bits=28``
+
 All of the above tools are available in the Clang/LLVM toolchain since 3.4.
 AddressSanitizer and ThreadSanitizer are available in recent versions of GCC,
 but are no longer actively maintained. MemorySanitizer is not available in GCC.