Adds mandatory features heading for phoenix layer

layers/+frameworks/phoenix/README.org

@@ -4,12 +4,16 @@
 
 * Table of Contents                                        :TOC_4_gh:noexport:
 - [[#description][Description]]
+  - [[#features][Features:]]
 - [[#install][Install]]
 - [[#key-bindings][Key bindings]]
 
 * Description
 This layer adds keybindings for [[https://github.com/tonini/alchemist.el][Alchemist]]'s already built in phoenix mode.
 
+** Features:
+  - Key bindings for navigation to files
+
 * Install
 To use this configuration layer, add it to your =~/.spacemacs=. You will need to
 add =phoenix= to the existing =dotspacemacs-configuration-layers= list in this