Add README with details on how to build.

README.md

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+This repository contains the website for [linebender.org](https://linebender.org). More details are
+available on the website.
+
+# Development
+
+The steps you need to get building the website locally are:
+
+ 1. [Install Zola](https://www.getzola.org/documentation/getting-started/installation/)
+ 2. Run `zola serve` in a terminal
+ 3. Go to the address logged to the terminal window (usually [localhost:1111](http://localhost:1111)).
+
+ # Deployment
+
+ This website is automatically deployed every time a PR is merged using Github actions.