Initial commit: Add README

Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>

README.md

+# Raspberry Pi - Apertis Custom Image Recipes
+
+This repository contains Apertis recipes customized for Raspberry Pi.
+
+Based on [Apertis Image Recipes](https://gitlab.apertis.org/infrastructure/apertis-image-recipes/)
+official repository. Check it out for further documentation on Apertis recipes.