added reference to the migration notes

Author: bbottema

README.md

@@ -76,6 +76,8 @@ v5.0.0.rc1 release candidate 1 (26-December-2017)
 
 This is currently a snapshot. To get access to it, add the [OSS snapshots repo](https://stackoverflow.com/a/7717234/441662) to your maven settings
 
+Also see the [migration notes](http://www.simplejavamail.org/#/migrate500)
+
 #### New features ####
 - [#116](https://github.com/bbottema/simple-java-mail/issues/116) You can now test the connection to the SMTP server
 - [#115](https://github.com/bbottema/simple-java-mail/issues/115) Create mailers with a very robust MailerBuilder API, able to ignore defaults as well

RELEASE.txt

@@ -10,6 +10,8 @@ v5.0.0 release candidate 1 (26-December-2017)
 
 This is currently a snapshot. To get access to it, add the OSS snapshots repo(https://stackoverflow.com/a/7717234/441662) to your maven settings
 
+Also see the migrating notes (http://www.simplejavamail.org/#/migrate500)
+
 New features
 - #116 You can now test the connection to the SMTP server
 - #115 Create mailers with a very robust MailerBuilder API, able to ignore defaults as well