Add SUPPORT.md

This name is also recognized by github:
https://docs.github.com/en/github/building-a-strong-community/adding-support-resources-to-your-project

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>

SUPPORT.md

@@ -0,0 +1,14 @@
+## Documentation
+
+Here are some useful sources of information about using Mbed TLS:
+
+- API documentation (see `make apidoc` or directly the header files);
+- the `docs` directory in the source tree;
+- the [Mbed TLS knowledge Base](https://tls.mbed.org/kb);
+- the [Mbed TLS mailing-list
+  archives](https://lists.trustedfirmware.org/pipermail/mbed-tls/).
+
+## Asking Questions
+
+If you can't find your answer in the above sources, please use the [Mbed TLS
+mailing list](https://lists.trustedfirmware.org/mailman/listinfo/mbed-tls).