RFR 8172183: Provide a javadoc description for jdk.dynalink module

[Sundararajan Athijegannathan]

Please review http://cr.openjdk.java.net/~sundar/8172183/webrev.00/ for 
https://bugs.openjdk.java.net/browse/JDK-8172183

Piggybacking couple of README cleanups in nashorn repo.
Thanks,
-Sundar