New app in WebMail category

I've added SquirellMail web-based IMAP client.

README.md

@@ -478,6 +478,7 @@ Comparison of NoSQL servers: http://kkovacs.eu/cassandra-vs-mongodb-vs-couchdb-v
 
 * [Mailpile](https://www.mailpile.is/) - A modern, fast web-mail client with user-friendly encryption and privacy features.
 * [Roundcube](http://roundcube.net/) - Browser-based IMAP client with an application-like user interface.
+* [SquirellMail](http://squirrelmail.org) - Another browser-based IMAP client.
 
 ## Web
 *Web servers.*