added description for polkadot-fast

Author: galin-s

README.md

@@ -104,7 +104,7 @@ nix shell github:metacraft-labs/nix-blockchain-development#solana
 | [kurtosis](https://github.com/kurtosis-tech/kurtosis)                                                                                    | A platform for packaging and launching ephemeral backend stacks with a focus on approachability for the average developer | x86_64-linux                                |
 | [pistache](https://github.com/pistacheio/pistache)                                                                                       | A high-performance REST toolkit written in C++                                                                            | x86_64-linux                                |
 | [polkadot](https://github.com/paritytech/polkadot)                                                                                       | Polkadot Node Implementation                                                                                              | x86_64-linux, x86_64-darwin, aarch64-darwin |
-| polkadot-fast                                         |                                                                                | x86_64-linux, x86_64-darwin, aarch64-darwin |
+| polkadot-fast                                         | fast-runtime = [ "polkadot-cli/fast-runtime" ]                                 | x86_64-linux, x86_64-darwin, aarch64-darwin |
 | [py-ecc](https://github.com/ethereum/py_ecc)          | Python implementation of ECC pairing and bn_128 and bls12_381 curve operations | x86_64-linux                                |
 | [rapidsnark](https://github.com/iden3/rapidsnark-old) | zkSnark proof generation written in C++ and intel assembly                     | x86_64-linux, x86_64-darwin                 |
 | rapidsnark-server                                     |                                                                                | x86_64-linux                                |