Update billing.mdx (#422)

Added two videos on adding GRT.

Author: marcusrein

website/pages/en/billing.mdx

@@ -41,6 +41,8 @@ While The Graph protocol operates on Ethereum Mainnet, [the billing contract](ht
 
 > This section is written assuming you already have GRT in your crypto wallet, and you're on Ethereum mainnet. If you don't have GRT, you can learn how to get GRT [here](#getting-grt).
 
+For a video walkthrough of adding GRT to your billing balance using a crypto wallet, watch this [video](https://youtu.be/4Bw2sh0FxCg).
+
 1. Go to the [Subgraph Studio Billing page](https://thegraph.com/studio/billing/).
 
 2. Click on the "Connect Wallet" button on the top right corner of the page. You'll be redirected to the wallet selection page. Select your wallet and click on "Connect".