docs: add note about gatsby v4 (#207)

gatsby-source-graphcms/README.md

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  <a href="https://gatsby-source-graphcms.vercel.app">Demo</a> • <a href="https://github.com/GraphCMS/gatsby-starter-graphcms-blog">gatsby-starter-graphcms-blog</a> • <a href="https://slack.graphcms.com">Join us on Slack</a> • <a href="https://app.graphcms.com">Login to GraphCMS</a> • <a href="https://twitter.com/GraphCMS">@GraphCMS</a>
 </p>
 
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-> **This documentation refers to the next version of `gatsby-source-graphcms`, which adds support to Gatsby v4. There are no breaking changes in the plugin, but make sure your website supports Gatsby v4.** <br /> > **If you are using Gatsby 3.x or 2.x, [click here](https://github.com/GraphCMS/gatsby-source-graphcms/tree/next).**
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 ## Installation
 
 > Note: Gatsby v4 requires Node.js >= 14.15.