Added a readme for the logo folder

doc/_static/logo/README.md

@@ -0,0 +1,7 @@
+# Yorc Logo
+
+[![License: CC BY 4.0](https://img.shields.io/badge/License-CC%20BY%204.0-lightgrey.svg)](https://creativecommons.org/licenses/by/4.0/)
+
+This folder contains Yorc logo material licensed under the Creative Commons Attribution 4.0 International licence.
+
+The font used in this logo is [Fresca](https://fonts.google.com/specimen/Fresca) by Fontstage licensed under the  [Open Font License](https://scripts.sil.org/cms/scripts/page.php?site_id=nrsi&id=OFL).