Adding configurations docs

docs/configuration/transport/email.md

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+# Email transport configuration
+
+ ## JSON
+```
+"Email": {
+  "to"        : "recipient@example.net;recipient2@example.net",
+  "from"      : "<name>from@example.net",
+  "encoding"  : "UTF-8",
+  "subject"   : "[Example.net] backup of the day",
+  "message"   : "Hi,\n\nYou can find your backup in this mail.\n\nRegards,\nMyself."
+}
+```
+ ## General 
+Emails are formated like this "email@domain.com" or like this "<name>email@domain.com"
+ 
+ ## Options
+to (mandatory)     = email addresses you want to send the backup to. Separated by semi-colon if more than one
+from (mandatory)   = person who send the backup by email.
+encoding           = email text encoding. Should stay with "UTF-8" or removed.
+subject (mandatory)= email subject
+message (mandatory)= email content sent as txt