Add README.md

Author: forell

README.md

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+The Fram Language
+=================
+
+This repository contains the source files for the Fram language documentation,
+which is currently hosted at [fram-lang.org](https://fram-lang.org/).
+
+Contributing
+------------
+
+The documentation is written in markdown and built using
+[mdBook](https://rust-lang.github.io/mdBook/).
+The [mdBook-KaTeX](https://github.com/lzanini/mdbook-katex) preprocessor is
+required to render math, but local builds should fail gracefully when it's not
+present.
+
+In order to build the site locally, run `mdbook build` in the root of the
+project's directory. You can also launch a local HTTP server and rebuild
+automatically on changes using the following command.
+```
+mdbook serve
+```
+
+Deployment
+----------
+
+The website is hosted using GitHub Pages. Deployment is performed automatically
+from the `master` branch of this repository; for details see [the workflow
+file](.github/workflows/deploy.yml).