Update SUPPORT.md

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>

Author: daverodgman

SUPPORT.md

@@ -6,6 +6,7 @@ Here are some useful sources of information about using Mbed TLS:
 - API documentation, see the [Documentation section of the
   README](README.md#documentation);
 - the `docs` directory in the source tree;
+- the [Mbed TLS knowledge Base](https://mbed-tls.readthedocs.io/en/latest/kb/);
 - the [Mbed TLS mailing-list
   archives](https://lists.trustedfirmware.org/archives/list/mbed-tls@lists.trustedfirmware.org/).