docs: add readme for regtest-launcher

regtest-launcher/README.md

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+# `regtest-launcher`
+
+Tiny Rust binary that launches a local Zcash regtest network (Zebrad & Zainod) and continuously mines blocks using Zebra’s RPC endpoints.
+
+## Overview
+
+- Starts a local validator + indexer using `zcash_local_net`.
+- Uses a provided miner transparent address **or** generates a fresh regtest transparent keypair.
+- Bootstraps the chain up to height **101**.
+- Then mines a new block every **5s**.
+- Prints indexer port + miner address, if generated.
+
+## Usage
+
+```bash
+Usage: regtest-launcher [OPTIONS]
+
+Options:
+      --activation-heights <ACTIVATION_HEIGHTS>
+          Comma-separated activation heights, e.g. "all=1,nu5=1000,nu6=off,nu6_1=off,nu7=off"
+
+          Keys: before_overwinter, overwinter, sapling, blossom, heartwood, canopy, nu5, nu6, nu6_1, nu7, all Values: u32 or off|none|disable
+
+          [default: all=1,nu7=off]
+
+      --miner-address <MINER_ADDRESS>
+          Optional miner address for receiving block rewards
+
+  -h, --help
+          Print help (see a summary with '-h')
+```