add docs to run parachain launch

scripts/parachain-launch/README.md

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+# parachain-launch
+
+
+
+# Introduction
+
+[Parachain-launch](https://github.com/open-web3-stack/parachain-launch) is a script that generates a docker compose file allowing you to launch a testnet of multiple blockchain nodes.
+
+The output directory already contains a generated docker-compose file so you can spin a testnet up without needing to install `parachain-launch`
+
+To get the testnet up and running run the below command
+
+```
+docker-compose -f output/docker-compose.yml up -d
+```
+
+To regenerate the compose file, first ensure `parachain-launch` is installed and run the following command
+
+```
+parachain-launch generate statemine.yml
+```
+
+# Requirements
+The only requirement to spin a testnet up is to have [docker-compose](https://docs.docker.com/compose/install/) installed, however to generate your own compose file then please follow the instructions to install `parachain-launch`