Update readme with 1.2.1 changes, add maven badge

Author: vlsi

README.md

@@ -1,8 +1,21 @@
 [![Build Status](https://travis-ci.org/vlsi/compactmap.svg?branch=master)](https://travis-ci.org/vlsi/compactmap)
+[![Maven Central](https://maven-badges.herokuapp.com/maven-central/com.github.vlsi.compactmap/compactmap/badge.svg)](https://maven-badges.herokuapp.com/maven-central/com.github.vlsi.compactmap/compactmap)
 
 CompactHashMap
 ==============
 
+Usage
+-----
+
+Add maven dependency:
+```xml
+<dependency>
+    <groupId>com.github.vlsi.compactmap</groupId>
+    <artifactId>compactmap</artifactId>
+    <version>1.2.1</version>
+</dependency>
+```
+
 About
 -----
 This is a memory efficient alternative to HashMap.
@@ -68,16 +81,20 @@ or (at your option) any later version.
 
 Change log
 ----------
+v1.2.1
+* Improvement: release to Maven Central
+* Improvement: fix EntrySet.remove method
+
 v1.2.0
-  Improvement: reduce size of hidden classes by using persistent dexx-collections.
-  Improvement: mavenize build
-  Switch to semantic versioning
+* Improvement: reduce size of hidden classes by using persistent dexx-collections.
+* Improvement: mavenize build
+* Switch to semantic versioning
 
 v1.1
-  Fix: #1 containKey returns true on non existing key
-  Fix: #2 size should account removed keys
-  Improvement: #3 Default values should be serialized as map
+* Fix: #1 containKey returns true on non existing key
+* Fix: #2 size should account removed keys
+* Improvement: #3 Default values should be serialized as map
 
 Author
 ------
-Vladimir Sitnikov <[email protected]>
+Vladimir Sitnikov <[email protected]>