Add the missing development guide

docs/development.md

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+## Development
+
+### Setting up
+
+First, get everything installed and configured with:
+
+```sh
+bin/setup
+```
+
+This installs the system packages Campfire needs (SQLite, ffmpeg), the right Ruby version (via [mise](https://mise.jdx.dev)), and the app's gems; prepares the database; and starts Redis (in a Docker container called `campfire-redis`, if it isn't already running locally).
+
+If you want to start over at any point, run:
+
+```sh
+bin/setup --reset
+```
+
+### Running the server
+
+Start the development server with:
+
+```sh
+bin/dev
+```
+
+You'll be able to access the app at http://localhost:3000.
+
+On first run you'll be guided through creating your admin account, and you can sign in with that account from then on.
+
+Note that Campfire needs Redis (for Action Cable, caching, and background jobs), so if you've restarted your machine or stopped the container, `docker start campfire-redis` will bring it back.
+
+### Web Push notifications
+
+Campfire uses VAPID (Voluntary Application Server Identification) keys to send browser push notifications. For notifications to work in development you'll need to generate a key pair and set these environment variables:
+
+- `VAPID_PRIVATE_KEY`
+- `VAPID_PUBLIC_KEY`
+
+You can generate a fresh pair (along with a secret key base, which you can ignore in development) by running:
+
+```sh
+script/admin/generate-secrets
+```
+
+### Running tests
+
+Run the unit tests with:
+
+```sh
+bin/rails test
+```
+
+And the browser-based system tests with:
+
+```sh
+bin/rails test:system
+```
+
+Before pushing your changes, you can run the full CI suite locally - style checks, security audits, and all the tests - with a single command:
+
+```sh
+bin/ci
+```
+
+### Contributing
+
+You are welcome - and encouraged - to modify Campfire to your liking.
+If you'd like to contribute your changes back, please read our [contributing guide](../CONTRIBUTING.md) first.