Add Sendgrid integration (incoming email handling)

docs/create-tasks-by-email.markdown

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+Create tasks by email
+=====================
+
+You can create tasks directly by sending an email.
+
+At the moment, Kanboard is integrated with 3 external services:
+
+- [Mailgun](http://kanboard.net/documentation/mailgun)
+- [Sendgrid](http://kanboard.net/documentation/sendgrid)
+- [Postmark](http://kanboard.net/documentation/postmark)
+
+These services handle incoming emails without having to configure any SMTP server.
+
+When an email is received, Kanboard receive the message on a specific end-point.
+All complicated works are already handled by those services.
+
+Incoming emails workflow
+------------------------
+
+1. You send an email to a specific address, by example **something+myproject@inbound.mydomain.tld**
+2. Your email is forwarded to the third-party SMTP servers
+3. The SMTP provider call the Kanboard webhook with the email in JSON or multipart/form-data formats
+4. Kanboard parse the received email and create the task to the right project
+
+Note: New tasks are automatically created in the first column.
+
+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 title
+- The email body becomes the task description (Markdown format)
+
+Incoming emails can be written in text or HTML formats.
+**Kanboard is able to convert simple HTML emails to Markdown**.
+
+Security and requirements
+-------------------------
+
+- The Kanboard webhook is protected by a random token
+- The sender email address must match a Kanboard user
+- The Kanboard project must have a unique identifier, by example **MYPROJECT**
+- The Kanboard user must be member of the project