Add(README): Add Stellar backend support in README.

Signed-off-by: Ilja von Hoessle <ilja@perun.network>

Author: iljabvh

README.md

@@ -77,6 +77,7 @@ There are multiple **blockchain backends** available. A backend is automatically
 - **Cardano.** The Cardano backend is available at [perun-cardano-backend](https://github.com/perun-network/perun-cardano-backend).
 - **NERVOS.** The NERVOS backend is available at [perun-ckb-backend](https://github.com/perun-network/perun-ckb-backend).
 - **Dfinity.** The Dfinity Internet Computer backend is available at [perun-icp-backend](https://github.com/perun-network/perun-icp-backend).
+- **Stellar.** The Stellar backend is available at [perun-stellar-backend](https://github.com/perun-network/perun-stellar-backend).
 - **Fabric.** The Hyperledger Fabric backend is available at [perun-fabric](https://github.com/perun-network/perun-fabric).
 - **SimBackend.** The SimBackend represents an ideal blockchain backend (`backend/sim`) implementation that can be used for testing.