Mention that RequestBuilder is mutable and not threadsafe, close AsyncHttpClient#238

Author: Stephane Landelle

api/src/main/java/com/ning/http/client/RequestBuilder.java

@@ -23,6 +23,8 @@
 
 /**
  * Builder for a {@link Request}.
+ * Warning: mutable and not thread-safe! Beware that it holds a reference on the Request instance it builds,
+ * so modifying the builder will modify the request even after it has been built.
  */
 public class RequestBuilder extends RequestBuilderBase<RequestBuilder> {