Update README.md

added Nethermind (C# client)

Author: tkstanczak

README.md

@@ -13,8 +13,8 @@ dotnet build --configuration Release "src/OpenVASP.CSharpClient.sln"
 Set up two environment variables:
 
 - WHISPER_RPC_URL - URL for whisper rpc (like geth node rpc https://127.0.0.1:8545)
-- ETHEREUM_RPC_URL - URL for eth rpc (like geth or parity node rpc https://127.0.0.1:8545)
+- ETHEREUM_RPC_URL - URL for eth rpc (like nethermind, geth or parity node rpc https://127.0.0.1:8545)
 
 ```
 dotnet test --configuration Release "src/OpenVASP.CSharpClient.sln"
-```
+```