Merge pull request #1007 from sudoyu/patch-2

Update index for Groovy/Gradle/Grails.

free-programming-books-ja.md

@@ -25,6 +25,9 @@
 * [Erlang](#erlang)
 * [Git](#git)
 * [Go](#go)
+* [Groovy](#groovy)
+    * [Gradle](#gradle)
+    * [Grails](#grails)
 * [Haskell](#haskell)
 * [Haxe](#haxe)
 * [iOS](#ios)