chore(gatsby-source-graphql): warn people that source-graphql is slow…

… for larger sites (#33970)

* fix(gatsby-source-graphql): warn people that source-graphql is slow for larger sites

As it doesn't yet support inc builds or preview.

This trips people up some time and we should be clear about the plugin's limitation.

* Update README.md

* Update README.md

Co-authored-by: Lennart <lekoarts@gmail.com>

packages/gatsby-source-graphql/README.md

@@ -5,6 +5,8 @@ Plugin for connecting arbitrary GraphQL APIs to Gatsby's GraphQL. Remote schemas
 - [Example website](https://using-gatsby-source-graphql.netlify.app/)
 - [Example website source](https://github.com/gatsbyjs/gatsby/tree/master/examples/using-gatsby-source-graphql)
 
+This source plugin does **not** support [incremental builds, cloud builds](https://support.gatsbyjs.com/hc/en-us/articles/360053099253-Gatsby-Builds-Full-Incremental-and-Cloud), and preview (on Gatsby Cloud). Please be aware that build times will be signficantly slower than regular source plugins as the size of your site goes past a hundred or so pages.
+
 ## Install
 
 `npm install gatsby-source-graphql`