Update README.md with paper

Signed-off-by: Felix Hoops <[email protected]>

README.md

@@ -5,6 +5,9 @@
 > [!WARNING]
 > This repository is intended for prototyping and as a reference implementation.
 
+> [!NOTE]
+> A preprint of a paper providing more bacckground information is available [on arXiv](https://arxiv.org/abs/2401.09488).
+
 > [!NOTE]
 > This software artifact was originally intended to support only Gaia-X Participant Credentials. It has since evolved to be fully configurable for almost any Verifiable Credential, almost any wallet application, and almost any current OIDC client.