Update build instructions to use gradlew.

Author: ilya-g

README.md

@@ -1,4 +1,4 @@
-# kotlinx.collections.immutable
+# Immutable Collections Library for Kotlin
 
 Immutable collection interfaces and implementation prototypes for Kotlin ([proposal](proposal.md))
 
@@ -8,8 +8,8 @@ To initialize submodules, use the following commands:
     git submodule init
     git submodule update
 
-Then you can build artifacts with:
+Then you can build and install artifacts to maven local with:
 
-    mvn install
+    gradlew build install
 
 Prototype implementation is based on [pcollections](http://pcollections.org/) (Copyright 2015 The pcollections Authors.)