Document the cargo features for Vello `{CPU, Common}` (#1007) Co-authored-by: Tom Churchman <thomas@kepow.org>

diff --git a/sparse_strips/vello_common/Cargo.toml b/sparse_strips/vello_common/Cargo.toml
index 3ffb49b..7915797 100644
--- a/sparse_strips/vello_common/Cargo.toml
+++ b/sparse_strips/vello_common/Cargo.toml

@@ -32,7 +32,7 @@
 simd = []
 # Get floating point functions from the standard library (likely using your target’s libc).
 std = ["peniko/std"]
-# Use floating point implementations from libm.v
+# Use floating point implementations from libm.
 libm = ["peniko/libm"]
 # Allow loading and saving Pixmap to PNG. 
 png = ["std", "dep:png"]

diff --git a/sparse_strips/vello_common/README.md b/sparse_strips/vello_common/README.md
index a500606..4537edc 100644
--- a/sparse_strips/vello_common/README.md
+++ b/sparse_strips/vello_common/README.md

@@ -22,6 +22,8 @@
 <!-- Intra-doc links used in lib.rs should be evaluated here.
 See https://linebender.org/blog/doc-include/ for related discussion. -->
 
+[crate::pixmap::Pixmap]: https://docs.rs/vello_common/latest/vello_common/pixmap/struct.Pixmap.html
+
 <!-- cargo-rdme start -->
 
 This crate includes common geometry representations, tiling logic, and other fundamental components used by both [Vello CPU][vello_cpu] and Vello Hybrid.
@@ -37,6 +39,19 @@
 
 ## Features
 
+- `std` (enabled by default): Get floating point functions from the standard library
+  (likely using your target's libc).
+- `libm`: Use floating point implementations from [libm].
+- `png` (enabled by default): Allow loading [`Pixmap`][crate::pixmap::Pixmap]s from PNG images.
+  Also required for rendering glyphs with an embedded PNG.
+  Implies `std`.
+- `simd`: Allows requesting SIMD execution modes.
+  Note that SIMD is not yet implemented.
+
+At least one of `std` and `libm` is required; `std` overrides `libm`.
+
+## Contents
+
 - Shared data structures for paths, tiles, and strips
 - Geometry processing utilities
 - Common logic for rendering stages
@@ -44,6 +59,7 @@
 This crate acts as a foundation for `vello_cpu` and `vello_hybrid`, providing essential components to minimize duplication.
 
 [vello_cpu]: https://crates.io/crates/vello_cpu
+[libm]: https://crates.io/crates/libm
 
 <!-- cargo-rdme end -->
 

diff --git a/sparse_strips/vello_common/src/lib.rs b/sparse_strips/vello_common/src/lib.rs
index 284d261..566f073 100644
--- a/sparse_strips/vello_common/src/lib.rs
+++ b/sparse_strips/vello_common/src/lib.rs

@@ -14,6 +14,19 @@
 //!
 //! ## Features
 //!
+//! - `std` (enabled by default): Get floating point functions from the standard library
+//!   (likely using your target's libc).
+//! - `libm`: Use floating point implementations from [libm].
+//! - `png` (enabled by default): Allow loading [`Pixmap`][crate::pixmap::Pixmap]s from PNG images.
+//!   Also required for rendering glyphs with an embedded PNG.
+//!   Implies `std`.
+//! - `simd`: Allows requesting SIMD execution modes.
+//!   Note that SIMD is not yet implemented.
+//!
+//! At least one of `std` and `libm` is required; `std` overrides `libm`.
+//!
+//! ## Contents
+//!
 //! - Shared data structures for paths, tiles, and strips
 //! - Geometry processing utilities
 //! - Common logic for rendering stages
@@ -21,6 +34,8 @@
 //! This crate acts as a foundation for `vello_cpu` and `vello_hybrid`, providing essential components to minimize duplication.
 //!
 //! [vello_cpu]: https://crates.io/crates/vello_cpu
+// If we ever use libm ourselves, it would be better to intra-doc-link to it here instead.
+//! [libm]: https://crates.io/crates/libm
 
 // LINEBENDER LINT SET - lib.rs - v3
 // See https://linebender.org/wiki/canonical-lints/

diff --git a/sparse_strips/vello_cpu/README.md b/sparse_strips/vello_cpu/README.md
index 0a6275d..e00ddc7 100644
--- a/sparse_strips/vello_cpu/README.md
+++ b/sparse_strips/vello_cpu/README.md

@@ -28,6 +28,7 @@
 [RenderContext::stroke_path]: https://docs.rs/vello_cpu/latest/vello_cpu/struct.RenderContext.html#method.stroke_path
 [RenderContext::glyph_run]: https://docs.rs/vello_cpu/latest/vello_cpu/struct.RenderContext.html#method.glyph_run
 [`RenderContext::render_to_pixmap`]: https://docs.rs/vello_cpu/latest/vello_cpu/struct.RenderContext.html#method.render_to_pixmap
+[`Pixmap`]: https://docs.rs/vello_cpu/latest/vello_cpu/struct.Pixmap.html
 
 <!-- cargo-rdme start -->
 
@@ -83,6 +84,11 @@
 assert_eq!(&result, expected_render);
 ```
 
+## Features
+
+- `png`(enabled by default): Allow loading [`Pixmap`]s from PNG images.
+  Also required for rendering glyphs with an embedded PNG.
+
 ## Caveats
 
 Vello CPU is an alpha for several reasons, including the following.

diff --git a/sparse_strips/vello_cpu/src/lib.rs b/sparse_strips/vello_cpu/src/lib.rs
index 9cc8551..6539331 100644
--- a/sparse_strips/vello_cpu/src/lib.rs
+++ b/sparse_strips/vello_cpu/src/lib.rs

@@ -53,6 +53,11 @@
 //! assert_eq!(&result, expected_render);
 //! ```
 //!
+//! ## Features
+//!
+//! - `png`(enabled by default): Allow loading [`Pixmap`]s from PNG images.
+//!   Also required for rendering glyphs with an embedded PNG.
+//!
 //! ## Caveats
 //!
 //! Vello CPU is an alpha for several reasons, including the following.