Added basic README.md with build instructions

README.md

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+# The Rust Language Reference
+
+This document is the primary reference for the Rust programming language.
+
+This document is not normative. It may include details that are specific
+to `rustc` itself, and should not be taken as a specification for the
+Rust language. We intend to produce such a document someday, but this is
+what we have for now.
+
+## Dependencies
+
+- rustc (the Rust compiler).
+- mdbook (use `cargo install mdbook` to install it).
+
+## Build steps
+
+First, go to the repository folder and test the code snippets to catch
+compilation errors:
+
+```bash
+cd reference
+mdbook test
+```
+
+And then generate the book:
+
+```bash
+mdbook build
+```
+
+The generated HTML will be in the `docs` folder.