README.md in semantic_logger-3.4.1 vs README.md in semantic_logger-4.0.0.beta1

@@ -48,9 +48,19 @@
 - NewRelic Appender: gem 'newrelic_rpm'
 - Syslog Appender: gem 'syslog_protocol' 0.9.2 or above
 - Syslog Appender to a remote syslogng server over TCP or UDP: gem 'net_tcp_client'
 - Splunk Appender: gem 'splunk-sdk-ruby'
 
+## V4 Upgrade notes
+
+The following changes need to be made when upgrading to V4:
+- Ruby V2.1 / JRuby V9.1 is now the minimum runtime version.
+- Replace calls to Logger#with_payload with SemanticLogger.named_tagged.
+- Replace calls to Logger#payload with SemanticLogger.named_tags.
+- Appenders now write payload data in a seperate :payload tag instead of mixing them.
+  directly into the root elements to avoid name clashes.
+- MongoDB Appender requires Mongo Ruby Client V2 or greater.
+
 ## Install
 
     gem install semantic_logger
 
 To configure a stand-alone application for Semantic Logger: