Add back cover feature description

README.md

@@ -8,6 +8,14 @@ Pip-Boy inspired phone holder built around the PINE64 PinePhone.
  - OpenScad
  - [BOSL library](https://github.com/revarbat/BOSL)
 
+## Features
+### PIP slideout back cover
+The back cover is composed of two interconnected components, printed in place (PIP). This allows the cover to expand and retract again, automatically connecting any connectors fixed to the cover.
+![Back model](feature_back_pip.png)
+
+The inner construction makes sure the back cover remains sturdy and the two pieces are inseparable. See the "xray":
+![Back model xray](feature_back_pip_xray.png)
+
 ## Build Instructions
 See [Instructions Readme](instructions/README.md).