docs: instructions on how to develop the docs website

docs/README.md

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+# Capsule Documentation
+
+1. Ensure to have [`yarn`](https://classic.yarnpkg.com/lang/en/docs/install/#debian-stable) installed in your path.
+2. `yarn install`
+
+## Local development
+
+```shell
+yarn develop
+```
+
+This will create a local webserver listening on `localhost:8080` with hot-reload of your local changes.