Update Migration Guide

By Guillaume Drouet:

* Third-party REST services provide a response with a '@' character in their JSON attributes, which need to be explicitely declared in POJO when mapping with jackason support in jersey 2 client. They are transparently removed with jersey 1 client.
* What to use in place of missing addParams method? Response was found here: [http://www.coderanch.com/t/634321/Web-Services/java/Jersey-translation-queryparams](http://www.coderanch.com/t/634321/Web-Services/java/Jersey-translation-queryparams)
* It was tricky to find how proxy settings on client can be migrated, a mix of changelogs between 2.x versions and javadoc ([https://jersey.java.net/apidocs/2.14/jersey/org/glassfish/jersey/client/ClientProperties.html](https://jersey.java.net/apidocs/2.14/jersey/org/glassfish/jersey/client/ClientProperties.html)) helped me. Maybe a dedicated section would be useful.
#### Affected Versions
[2.14]