Add readme for web extension

packages/extension/README.md

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+# @padloc/extension
+
+The Padloc browser extension.
+
+## Setup
+
+The `@padloc/extension` package is meant to be used from within the
+[Padloc monorepo](../../README.md).
+
+```sh
+git clone git@github.com:padloc/padloc.git
+cd padloc
+npm ci
+cd packages/extension
+```
+
+## Building
+
+To build an unpacked version of the web extension, simply run the following from
+within the package directory.
+
+```sh
+npm run build
+```
+
+The resulting build can be fund in the `dist` folder.
+
+### Build options
+
+All build options are provided as environment variables:
+
+| Variable Name   | Description                                        | Default  |
+| --------------- | -------------------------------------------------- | -------- |
+| `PL_SERVER_URL` | URL to the [server component](../server/README.md) | `./dist` |
+
+### Installing an unpacked extension
+
+Once built, the easiest way to install and use the extension is to install it as
+an "unpacked extension". Steps vary from browser to browser:
+
+Google Chrome:
+https://developer.chrome.com/docs/extensions/mv3/getstarted/#unpacked
+
+Firefox:
+https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Your_first_WebExtension#installing
+
+## Development
+
+For development instructions, please refer to the
+[monorepo readme](../../README.md#development).
+
+## Contributing
+
+For info on how to contribute to Padloc, please refer to the
+[monorepo readme](../../README.md#contributing).