Add Sendgrid integration (incoming email handling)

docs/sendgrid.markdown

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+Sendgrid
+========
+
+You can use the service [Sendgrid](https://sendgrid.com/) to create tasks directly by email.
+
+This integration works with the [Parse API of Sendgrid](https://sendgrid.com/docs/API_Reference/Webhooks/parse.html).
+Kanboard use a webhook to handle incoming emails.
+
+The [incoming email workflow is described here](http://kanboard.net/documentation/email-tasks).
+
+Sendgrid configuration
+----------------------
+
+1. Create a new domain or subdomain (by example **inbound.mydomain.tld**) with a MX record that point to **mx.sendgrid.net**
+2. Add your domain and the Kanboard webhook url to [the configuration page in Sendgrid](https://sendgrid.com/developer/reply)
+
+The Kanboard webhook url is displayed in **Settings > Integrations > Sendgrid**
+
+Kanboard configuration
+----------------------
+
+1. Be sure that your users have an email address in their profiles
+2. Assign a project identifier to the desired projects: **Project settings > Edit**
+3. Try to send an email to your project