docs: add install browser extension

docs/install-browser-extension.md

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+# The Browser Extension of Slash
+
+Slash provides a browser extension to help you use your shortcuts in the search bar to go to the corresponding URL.
+
+## How to use
+
+### Generate an access token
+
+1. Go to your Slash instance and sign in with your account.
+
+2. Go to the settings page and click on the "Create" button to create an access token.
+
+   ![](./assets/extension-usage/create-access-token.png)
+
+3. Copy the access token and save it somewhere safe.
+
+   ![](./assets/extension-usage/copy-access-token.png)
+
+### Install the extension
+
+> **Note**: The extension is not published to the Chrome Web Store yet. You can install it from the source code.
+
+For Chromuim based browsers, you can download the packed extension from the [resources](https://github.com/boojack/slash/tree/main/extension/resources).
+
+For Firefox, we don't support the Firefox Add-ons platform yet. And we are working on it.
+
+### Configure the extension
+
+1. Click on the extension icon and click on the "Settings" button.
+
+   ![](./assets/extension-usage/extension-setting-button.png)
+
+2. Enter your Slash's domain and paste the access token you generated in the previous step.
+
+   ![](./assets/extension-usage/extension-setting-page.png)
+
+3. Click on the "Save" button to save the settings.
+
+4. Click on the extension icon again, you will see a list of your shortcuts.
+
+   ![](./assets/extension-usage/extension-screenshot.png)
+
+### Use your shortcuts in the search bar
+
+You can use your shortcuts in the search bar of your browser. For example, if you have a shortcut named `gh` for [GitHub](https://github.com), you can type `s/gh` in the search bar and press `Enter` to go to [GitHub](https://github.com).
+
+![](./assets/extension-usage/shortcut-url.png)