Added docs for cli usage and adding new commands (#68)

docs/content/doc/development/cli.md

@@ -0,0 +1,34 @@
+---
+date: "2019-03-31:00:00+01:00"
+title: "Adding new cli commands"
+draft: false
+type: "doc"
+menu:
+  sidebar:
+    parent: "development"
+---
+
+# Adding new cli commands
+
+All cli-related functions are located in `pkg/cmd`.
+Each cli command usually calls a function in another package.
+For example, the `vikunja migrate` command calls `migration.Migrate()`. 
+
+Vikunja uses the amazing [cobra](https://github.com/spf13/cobra) library for its cli.
+Please refer to its documentation for informations about how to use flags etc.
+
+To add a new cli command, add something like the following:
+
+{{< highlight golang >}}
+func init() {
+	rootCmd.AddCommand(myCmd)
+}
+
+var myCmd = &cobra.Command{
+	Use:   "My-command",
+	Short: "A short description about your command.",
+	Run: func(cmd *cobra.Command, args []string) {
+		// Call other functions
+	},
+}
+{{</ highlight >}}