(doc) initial S/MIME doc

Documentation/SOGoInstallationGuide.asciidoc

@@ -2902,6 +2902,19 @@ SOGoMaximumSyncInterval = 3540;
 SOGoInternalSyncInterval = 60;
 ----
 
+S/MIME Support in SOGo
+----------------------
+
+SOGo supports S/MIME email signing and encryption. When receiving S/MIME signed emails, SOGo automatically extracts the
+PKCS (Public-Key Cryptography Standard) #7 signature and stores it in the user's personal address book for the contact
+associated to the email address of the email's sender. This certificate will allow the user to send encrypted emails to
+these recipients.
+
+User that wish to send signed emails must upload their certificate in PKCS #12 format. When doing so,
+SOGo will convert the PKCS #12 file into the PEM format and store it in the user's preferences.
+
+If you are looking for a free S/MIME certificate from a well-known CA, please have a look at Comodo: https://www.comodo.com/home/email-security/free-email-certificate.php
+
 
 Using SOGo
 ----------