add source code link for sylpheed

- ref. #416

README.md

@@ -483,7 +483,7 @@ _See also: [IT Asset Management]([Ralph](#it-asset-management))_
 - [Claws Mail](http://www.claws-mail.org/) - Old school email client (and news reader), based on GTK+. ([Source Code](https://git.claws-mail.org/?p=claws.git;a=tree))
 - [ImapSync](http://imapsync.lamiral.info/) – Simple IMAP migration tool for copying mailboxes to other servers. ([Source Code](https://github.com/imapsync/imapsync))
 - [Mutt](http://www.mutt.org/) - Small but very powerful text-based mail client. ([Source Code](https://gitlab.com/muttmua/mutt))
-- [Sylpheed](http://sylpheed.sraoss.jp/en/) – Still developed predecessor to Claws Mail, lightweight mail client.
+- [Sylpheed](http://sylpheed.sraoss.jp/en/) – Still developed predecessor to Claws Mail, lightweight mail client. ([Source Code](https://github.com/sylpheed-mail/sylpheed))
 - [Thunderbird](https://www.mozilla.org/de/thunderbird/) - Free email application that's easy to set up and customize.