Update readme.md with JDK compatibility

Author: joriscode

readme.md

@@ -7,3 +7,9 @@
 mdoc is a markdown documentation tool for Scala inspired by
 [tut](https://github.com/tpolecat/tut). Visit https://scalameta.org/mdoc to
 learn more about mdoc.
+
+## JDK compatibility
+| JDK  | Release        |
+| ---- | ---------------|
+| 8    | Up to `v2.6.1` |
+| 11+  | _latest_       |