Update readme with proguard instructions.

Author: dnkoutso

README.md

@@ -1,5 +1,5 @@
 Picasso
-========
+=======
 
 A powerful image downloading and caching library for Android
 
@@ -23,6 +23,16 @@ Download [the latest JAR][2] or grab via Maven:
 ```
 
 
+ProGuard
+--------
+
+If you are using ProGuard make sure you add the following option:
+
+```
+-dontwarn com.squareup.okhttp.**
+```
+
+
 
 License
 --------