Improve readme

README.md

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 **Name:** `hmarr/auto-approve-action`
 
-**Requires the `GITHUB_TOKEN` environment variable checkbox to be checked.**
+**🔒 Requires the `GITHUB_TOKEN` environment variable checkbox to be checked.**
 
 Automatically approve GitHub pull requests. Combine with the filter action to
 only auto-approve certain users. For example, to auto-approve
@@ -13,4 +13,8 @@ for the filter action.
   <img src="docs/approve-workflow.png" width="200">
 </p>
 
+## Why?
+
+GitHub lets you prevent merges of unapproved pull requests. However, it's occasionally useful to selectively circumvent this restriction - for instance, some people want Dependabot's automated pull requests to not require approval. Combining this action with the filter action lets you automatically approve certain pull requests.
+
 [dependabot]: https://github.com/marketplace/dependabot