Add Mailgun integration (incoming emails)

docs/postmark.markdown

@@ -21,7 +21,7 @@ Email format
 
 - The local part of the email address must use the plus separator, by example **kanboard+project123**
 - The string defined after the plus sign must match a project identifier, by example **project123** is the identifier of the project **Project 123**
-- The email subject becomes the task subject
+- The email subject becomes the task title
 - The email body becomes the task description (Markdown format)
 
 Incoming emails can be written in text or HTML formats.