Add basic docs for extras classes, fixes PHPMailer#240

extras/README.md

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+#PHPMailer Extras
+
+These classes provide optional additional functions to PHPMailer.
+
+These are not loaded by the PHPMailer autoloader, so in some cases you may need to `require` them yourself before using them.
+
+##HTML2Text
+
+This class was written by Jon Abernathy and provides a simple conversion of HTML to plain-text, while attempting to preserve some aspects of the formatting. It is used in PHPMailer if you set the `advanced` parameter to `true` in either the `msgHTML()` or `html2text` methods of PHPMailer.
+
+##EasyPeasyICS
+
+This class was originally written by Manuel Reinhard and provides a simple means of generating ICS/vCal files that are used in sending calendar events. PHPMailer does not use it diorectly, but you can use it to generate content appropriate for placing in the `Ical` property of PHPMailer. The PHPMailer project is now its official home as Manuel has given permission for that and is no longer maintaining it himself.
+
+##htmlfilter
+
+This class by Konstantin Riabitsev and Jim Jagielski implements HTML filtering to remove potentially malicious tags, such as `<script>` or `onclick=` attributes that can result in XSS attacks. This is a simple filter and is not as comprehensive as [HTMLawed](http://www.bioinformatics.org/phplabware/internal_utilities/htmLawed/) or [HTMLPurifier](http://htmlpurifier.org), but it's easier to use and considerably better than nothing! PHPMailer does not use it directly, but you may want to apply it to user-supplied HTML before using it as a message body.
+
+##NTLM_SASL_client
+
+This class by Manuel Lemos (bundled with permission) adds the ability to authenticate with Microsoft Windows mail servers that use NTLM-based authentication. It is used by PHPMailer if you send via SMTP and set the `AuthType` property to `NTLM`; you will also need to use the `Realm` and `Workstation` properties. The original source is [here](http://www.phpclasses.org/browse/file/7495.html).