README: Add Fabric to backend list (#377)

Signed-off-by: Philipp-Florens Lehwalder <philipp@perun.network>

Signed-off-by: Philipp-Florens Lehwalder <philipp@perun.network>

Author: cryptphil

README.md

@@ -74,6 +74,7 @@ There are multiple **blockchain backends** available. A backend is automatically
 - **Ethereum.** The Ethereum backend is available at [perun-eth-backend](https://github.com/perun-network/perun-eth-backend/).
 - **Polkadot.** The Polkadot backend is available at [perun-polkadot-backend](https://github.com/perun-network/perun-polkadot-backend).
 - **Cosmos.** The Cosmos backend is available at [perun-cosmwasm-backend](https://github.com/hyperledger-labs/perun-cosmwasm-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.
 
 **Logging and networking** capabilities can also be injected by the user.