feat: added xDAI Chain MetaMask tutorial

Author: hbarcelos

users/_assets/metamask-xdai-chain/00_metamask_icon.png

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+# How to xDAI Chain to MetaMask
+
+Guide to the process of adding a new chain to MetaMask. This will allow you to use Kleros dapps on xDAI.
+
+## 0. Install MetaMask
+
+Skip if you already have MetaMask installed.
+
+1. Go to [MetaMask downloads page](https://metamask.io/download.html).
+2. Click on "Install MetaMask for <YOUR BROWSER>".
+3. Follow the instructions to install the MetaMask extension in your browser.
+
+## 1. Open MetaMask Settings
+
+1. Click on the MetaMask icon in your browser toolbar then select "Expand View":
+
+    ![Metamask Icon](./_assets/metamask-add-xdai/00_metamask_icon.png)
+
+1. On the new tab that have just opened, click on the colorful avatar circle on the top and then select "Settings":
+
+    ![Go to Settings](./_assets/metamask-add-xdai/01_go_to_settings.png)
+
+1. Select "Networks", then click on "Add Network":
+
+    ![Networks](./_assets/metamask-add-xdai/02_add_network.png)
+
+1. Fill the text fields with the following info:
+
+    | Field                      | Value
+    | :---                       | :---
+    | Network Name               | `Sokol`
+    | New RPC URL                | `https : //sokol.poa.network`
+    | Chain ID                   | `77`
+    | Currency Symbol (optional) | `SPOA`
+    | Currency Symbol (optional) | `https : //blockscout.com/poa/sokol`
+
+1. Click "Save"
+
+    ![xDAI Params](./_assets/metamask-add-xdai/02a_xdai_params.png)
+
+---
+
+The new network is automatically selected. To switch to this network in the future, simply click on the MetaMask icon in your browser toolbar, click on the networks dropdown button and then select the desired network:
+
+![Select Netwwork](./_assets/metamask-add-xdai/03_select_xdai.png)