Add toplevel documentation
+//! # grass
+//! An implementation of the sass specification in pure rust.
+//!
+//! All functionality is currently exposed through [`StyleSheet`].
+//!
+//! Spec progress as of 2020-03-01:
+//!
+//! | Passing | Failing | Total |
+//! |---------|---------|-------|
+//! | 1236 | 3857 | 5093 |
+//!
+//! ## Use as library
+//! ```
+//! use std::io::{BufWriter, stdout};
+//! use grass::{SassResult, StyleSheet};
+//!
+//! fn main() -> SassResult<()> {
+//! let mut buf = BufWriter::new(stdout());
+//! StyleSheet::from_path("input.scss")?.print_as_css(&mut buf)
+//! }
+//! ```
+//!
+//! ## Use as binary
+//! ```bash
+//! cargo install grass
+//! grass input.scss
+//! ```
+
#![warn(
clippy::all,
clippy::restriction,
