Add configuration instructions in groovy to readme

Thought this would be helpful for people who're still groovy, like me. The name of the extension can only be found by digging inside the sources.

README.md

@@ -106,11 +106,19 @@ The Gradle plugin allows configuring the functions which should be transformed
 with a list of fully-qualified function names.
 
 ```kotlin
+// Kotlin DSL
 configure<com.bnorm.power.PowerAssertGradleExtension> {
   functions = listOf("kotlin.assert", "kotlin.test.assertTrue")
 }
 ```
 
+```groovy
+// Groovy
+kotlinPowerAssert {
+  functions = ["kotlin.assert", "kotlin.test.assertTrue"]
+}
+```
+
 ## Kotlin IR
 
 Using this compiler plugin only works if the code is compiled using Kotlin