Adds Netlify badge

partially a commit to start generating Netlify previews on arbitrary branches!

README.md

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-    <a href="https://badge.fury.io/rb/just-the-docs"><img src="https://badge.fury.io/rb/just-the-docs.svg" alt="Gem version"></a> <a href="https://github.com/just-the-docs/just-the-docs/actions?query=workflow%3A%22main+branch+CI%22"><img src="https://github.com/just-the-docs/just-the-docs/workflows/main%20branch%20CI/badge.svg" alt="Build status"></a>
+    <a href="https://badge.fury.io/rb/just-the-docs"><img src="https://badge.fury.io/rb/just-the-docs.svg" alt="Gem version"></a> <a href="https://github.com/just-the-docs/just-the-docs/actions?query=workflow%3A%22main+branch+CI%22"><img src="https://github.com/just-the-docs/just-the-docs/workflows/main%20branch%20CI/badge.svg" alt="Build status"></a> <a href="https://app.netlify.com/sites/just-the-docs/deploys"><img src="https://api.netlify.com/api/v1/badges/9dc0386d-c2a4-4077-ad83-f02c33a6c0ca/deploy-status" alt="Netlify Status"></a>
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