readme: added description

README.md

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 [![PHP version](https://badge.fury.io/ph/fphammerle%2Fyii2-client-cert-auth.svg)](https://badge.fury.io/ph/fphammerle%2Fyii2-client-cert-auth)
 [![Build Status](https://travis-ci.org/fphammerle/yii2-client-cert-auth.svg?branch=master)](https://travis-ci.org/fphammerle/yii2-client-cert-auth)
 
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+yii2 extension for automatic login via TLS/SSL/HTTPS client certificates
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 ## Setup
 
 ### 1. Configure Webserver