Update README.md

Add logo at the top and improve description.

README.md

@@ -1,8 +1,10 @@
-# RetroMeet
+<h1><picture>
+  <img alt="RetroMeet logo" src="https://github.com/retromeet/core/blob/main/app/assets/images/retromeet_long.png?raw=true">
+</picture></h1>
 
 RetroMeet is a free, open-source dating and friend-finding application. RetroMeet has a philosophy of giving more space to give more information about yourself and to help find people who share common interests with you. The whole philosophy of RetroMeet is described in [the philosophy](https://join.retromeet.social/philosophy).
 
-This repository contains the RetroMeet core. This component provides an API to access the app functionalities without any external interface.
+This repository contains the RetroMeet core. This component provides an API to access the app functionalities, plus the authentication/authorization flows using OAuth.
 
 ## Development