Huge improvements for docs (#58)

docs/content/doc/practical-instructions/mail.md

@@ -0,0 +1,84 @@
+---
+date: "2019-02-12:00:00+02:00"
+title: "Mailer"
+draft: false
+type: "doc"
+menu:
+  sidebar:
+    parent: "practical instructions"
+---
+
+# Mailer
+
+This document explains how to use the mailer to send emails and what to do to create a new kind of email to be sent.
+
+## Sending emails
+
+**Note:** You should use mail templates whenever possible (see below).
+
+To send an email, use the function `mail.SendMail(options)`. The options are defined as follows:
+
+{{< highlight golang >}}
+type Opts struct {
+	To          string // The email address of the recipent
+	Subject     string // The subject of the mail
+	Message     string // The plaintext message in the mail
+	HTMLMessage string // The html message 
+	ContentType ContentType // The content type of the mail. Can be either mail.ContentTypePlain, mail.ContentTypeHTML, mail.ContentTypeMultipart. You should set this according to the kind of mail you want to send.
+	Boundary    string
+	Headers     []*header // Other headers to set in the mail.
+}
+{{< /highlight >}}
+
+## Sending emails based on a template
+
+For each mail with a template, there are two email templates: One for plaintext emails, one for html emails.
+
+These are located in the `templates/mail` folder and follow the conventions of `template-name.{plain|hmtl}.tmpl`,
+both the plaintext and html templates are in the same folder.
+
+To send a mail based on a template, use the function `mail.SendMailWithTemplate(to, subject, tpl string, data map[string]interface{})`.
+`to` and `subject` are pretty much self-explanatory, `tpl` is the name of the template, without `.html.tmpl` or `.plain.tmpl`. 
+`data` is a map you can pass additional data to your template.
+
+#### Sending a mail with a template
+
+A basic html email template would look like this:
+
+{{< highlight go-html-template >}}
+{{template "mail-header.tmpl" .}}
+<p>
+    Hey there!<br/>
+    This is a minimal html email example.<br/>
+    {{.Something}}
+</p>
+{{template "mail-footer.tmpl"}}
+{{< /highlight >}}
+
+And the corresponding plaintext template:
+
+{{< highlight go-text-template >}}
+Hey there!
+
+This is a minimal html email example.
+
+{{.Something}}
+{{< /highlight >}}
+You would then call this like so:
+
+{{< highlight golang >}}
+data := make(map[string]interface{})
+data["Something"] = "I am some computed value"
+to := "test@example.com"
+subject := "A simple test mail"
+tpl := "demo" // Assuming you saved the templates as demo.plain.tmpl and demo.html.tmpl
+mail.SendMailWithTemplate(to, subject, tpl, data)
+{{< /highlight >}}
+
+The function does not return an error. If an error occures when sending a mail, it is logged but not returned because sending the mail happens asinchrounly.
+
+Notice the `mail-header.tmpl` and `mail-footer.tmpl` in the template. These populate some basic css, a box for your content and the vikunja logo.
+All that's left for you is to put the content in, which then will appear in a beautifully-styled box.
+
+Remeber, these are email templates. This is different from normal html/css, you cannot use whatever you want (because most of the clients are wayyy to outdated).
+