Add eml-codec, imap-codec, smtp-message to README

[superboum]

From README.md, I read:

Want to add your parser here? Create a pull request for it!

This PR attempts to add 3 projects using nom:

• eml-codec, a parser written with nom for Internet Message Format (IMF, RFC5322) and MIME (RFC2045)
• imap-codec, a parser for the IMAP protocol written by @duesee with nom, that I am using in my projects
• smtp-message, a parser for the SMTP protocol written by @ekleog written with nom, that I am using in my projects

[Geal]

great, thanks!