chore: Add GatsbyConf 2022 banner to README (#34782)

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

README.md

@@ -1,3 +1,7 @@
+[![A dark purple background with hints of stars. On top of that it says "GatsbyConf 2022" and "March 2-3". The letters "2022" are big, playful, light purple, and have illustrations of stars, people in orbs around them.](https://user-images.githubusercontent.com/3477155/153261605-43e1075b-7889-435b-bd34-a5980b629843.jpg)](https://gatsbyconf.com?utm_source=github&utm_medium=readme-banner&utm_campaign=gatsbyconf22-repo-banner)
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+---
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 <p align="center">
   <a href="https://www.gatsbyjs.com">
     <img alt="Gatsby" src="https://www.gatsbyjs.com/Gatsby-Monogram.svg" width="60" />