Link to readme of MDC propagation module.

Author: sjoerdtalsma

README.md

@@ -22,6 +22,7 @@ The following `ThreadLocal`-based contexts are currently supported
 out of the box by this context-propagation library:
 
 - [ServletRequest contexts][servletrequest propagation]
+- [Slf4J Mapped Diagnostic Context (MDC)][mdc propagation]
 - [OpenTracing Span contexts][opentracing span propagation]
 - _yours?_ Feel free to create an issue or pull-request
   if you believe there's a general context that was forgotten. 
@@ -70,5 +71,6 @@ public class DummyContextManager implements ContextManager<String> {
   [maven]: http://search.maven.org/#search%7Cga%7C1%7Cg%3A%22nl.talsmasoftware.context%22%20AND%20a%3A%22context-propagation%22
 
   [servletrequest propagation]: servletrequest-propagation
+  [mdc propagation]: mdc-propagation
   [opentracing span propagation]: opentracing-span-propagation
   [default constructor]: https://en.wikipedia.org/wiki/Nullary_constructor