8251551: Use .md filename extension for README

Reviewed-by: mr, ihse, darcy

Author: edvbld

README

@@ -0,0 +1,11 @@
+# Welcome to the JDK!
+
+For build instructions please see the
+[online documentation](https://openjdk.java.net/groups/build/doc/building.html),
+or either of these files:
+
+- [doc/building.html](doc/building.html) (html version)
+- [doc/building.md](doc/building.md) (markdown version)
+
+See <https://openjdk.java.net/> for more information about
+the OpenJDK Community and the JDK.