add usage example

benchmarks/src/main/kotlin/UcfsRunExample.kt

@@ -0,0 +1,14 @@
+import org.ucfs.Java8
+import org.ucfs.parser.Gll
+
+fun main(){
+    // get RSM start state
+    val startState = Java8().rsm
+    // get linear graph from source code
+    val text = "package a; class X {}"
+    val tokens = getTokenStream(text)
+    // parse (intersect graph and rsm)
+    val gll = Gll.gll(startState, tokens)
+    // make sure that sppf isn't empty
+    assert(gll.parse().first != null) { "can't build sppf" }
+}