update readme

README.md

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 This is the MassaStation plugin that implements the Massa wallet features.
 
+## Add a token in the wallet
+
+As a developer, you can add a token in the wallet by following these steps:
+
+- Open a pull request in the ui-kit to add the icon [here](https://github.com/massalabs/ui-kit/tree/main/src/components/Icons/Svg/FT) and add a line [here](https://github.com/massalabs/ui-kit/blob/main/src/components/Icons/Svg/FT/tokenIcons.tsx#L100)
+- Open a pull request in this repository to add the token [here](https://github.com/massalabs/station-massa-wallet/blob/4aa5d5b7885d5d5227724f5114de99c409701a6e/pkg/assets/default_assets.go#L121) and update the ui-kit with `npm update @massalabs/react-ui-kit`
+
 ## Developer guide
 
 This section helps developer getting started.