Add Gitter to README

README.adoc

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+image::https://badges.gitter.im/Join%20Chat.svg[Gitter,link=https://gitter.im/spring-projects/spring-security?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge]
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 = Spring Security
 
 Spring Security provides security services for the http://docs.spring.io[Spring IO Platform]. Spring Security 3.1 requires Spring 3.0.3 as