Add using changesets in the contributing section of readme

Signed-off-by: William So <[email protected]>

README.md

@@ -22,6 +22,16 @@ This is NOT an installable plugin.
 
 Contributions are welcome!
 
+This project uses [`changesets`](https://github.com/changesets/changesets) to manage the changelog. When creating a pull request, please [add a changeset](https://github.com/changesets/changesets/blob/main/docs/intro-to-using-changesets.md#adding-changesets) describing the changes. Add multiple changesets if your pull request changes several things. End each changeset with `([PR number](PR link) by [author username](author link))`. For example, the newly created file under the directory `.changeset` should look like:
+
+```Markdown
+---
+"example": patch
+---
+
+This is an example change. ([GH#1](https://github.com/ghost/example/pull/1) by [@ghost](https://github.com/ghost))
+```
+
 ### Todos
 
 The todos here, ordered alphabetically, are things planned for the plugin. There are no guarantees that they will be completed. However, we are likely to accept contributions for them.