chore: update notable users in Readme

Add [Ursa](https://github.com/fleek-network/ursa) by fleek to the notable user's section in Readme.

Pull-Request: #3493.

Author: b0xtch

README.md

@@ -101,3 +101,4 @@ used and maintained by [Actyx](https://www.actyx.com).
 - [Subspace](https://github.com/subspace/subspace) - Subspace Network reference implementation
 - [Substrate](https://github.com/paritytech/substrate) - Framework for blockchain innovation,
 used by [Polkadot](https://www.parity.io/technologies/polkadot/).
+- [Ursa](https://github.com/fleek-network/ursa) - Decentralized Content Delivery & Edge Compute.