Document the `multithreading` features in `vello_cpu`

sparse_strips/vello_cpu/Cargo.toml

diff --git a/sparse_strips/vello_cpu/Cargo.toml b/sparse_strips/vello_cpu/Cargo.toml
index dfb0c75..bc0a571 100644
--- a/sparse_strips/vello_cpu/Cargo.toml
+++ b/sparse_strips/vello_cpu/Cargo.toml
@@ -31,6 +31,7 @@
 libm = ["vello_common/libm", "dep:libm"]
 # Allow loading Pixmap from PNG, and drawing png glyphs.
 png = ["vello_common/png"]
+# Enable multi-threaded rendering (currently only a part of the pipeline can be parallelized).
 multithreading = ["std", "dep:rayon", "dep:thread_local"]
 
 [lints]

sparse_strips/vello_cpu/README.md

diff --git a/sparse_strips/vello_cpu/README.md b/sparse_strips/vello_cpu/README.md
index 184c950..88920e3 100644
--- a/sparse_strips/vello_cpu/README.md
+++ b/sparse_strips/vello_cpu/README.md
@@ -92,6 +92,7 @@
 - `libm`: Use floating point implementations from [libm].
 - `png`(enabled by default): Allow loading [`Pixmap`]s from PNG images.
   Also required for rendering glyphs with an embedded PNG.
+- `multithreading`: Enable multi-threaded rendering (currently only a part of the pipeline can be parallelized).
 
 At least one of `std` and `libm` is required; `std` overrides `libm`.