Add README for docs

docs/README.md

@@ -0,0 +1,14 @@
+# KxOS Documentation
+
+The documentation is rendered as a book with [mdBook](https://rust-lang.github.io/mdBook/),
+which can be installed with `cargo`.
+
+```bash
+cargo install mdbook
+```
+
+To build the book and read it in your default browser, run the following command.
+
+```bash
+mdbook serve --open
+```