feat(custom-tokens.mdx): add info box recommending the use of the fun…

…gible token standard.

docs/zkapps/writing-a-zkapp/feature-overview/custom-tokens.mdx

@@ -17,6 +17,14 @@ keywords:
   - tokencontract
 ---
 
+:::info
+
+If you want to create a fungible token you can use the 
+[fungible token standard](https://github.com/MinaFoundation/mina-fungible-token/blob/main/documentation/SUMMARY.md) which is built on top of the `TokenContract` class.  
+
+
+:::info
+
 # Custom Tokens
 
 Blockchain applications have various use cases for custom tokens, including a real-world financial asset, stake in an on-chain protocol, or even skill points in a game.