Add README with setup and usage

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: turian

README.md

@@ -1,3 +1,25 @@
 <p align="center">
   <img src="asset/mascot.png" alt="Fastmail JMAP CLI" width="300">
 </p>
+
+# Fastmail JMAP CLI
+
+Read-only CLI for agents to access Fastmail via JMAP.
+
+## Setup
+
+```bash
+uv sync
+export FASTMAIL_READONLY_API_TOKEN="fmu1-..."  # from Fastmail Settings → Integrations
+```
+
+## Usage
+
+```bash
+fastmail-cli help                              # list commands
+fastmail-cli email.query --limit 5             # recent emails
+fastmail-cli email.get --ids '["M123"]'        # get by ID
+fastmail-cli mailbox.query                     # list mailboxes
+```
+
+All output is JSON with `ok`, `command`, `meta`, and `data`/`error` fields.