README: mention that we can use package.metadata to select CPUs

Author: ronnychevalier

README.md

@@ -11,11 +11,20 @@ Cargo subcommand to build multiple versions of the same binary, each with a diff
 
 `cargo-multivers` builds multiple versions of the binary of a Rust package.
 Each version is built with a set of CPU features (e.g., `+cmpxchg16b,+fxsr,+sse,+sse2,+sse3`) from a CPU (e.g., `ivybridge`) supported by the target (e.g., `x86_64-pc-windows-msvc`).
-It does not build the powerset of the CPU features, but only a subset: from the list of CPU known to `rustc` for a given target, it fetches each set of CPU features and filters out
+
+By default, it lists the CPUs known to `rustc` for a given target, then it fetches each set of CPU features and filters out
 the duplicates.
+You can also add a section to your `Cargo.toml` to set the allowed list of CPUs for your package.
+For example, for `x86_64` you could add:
+
+```toml
+[package.metadata.multivers.x86_64]
+cpus = ["generic", "alderlake", "skylake", "sandybridge", "ivybridge"]
+```
+
 After building the different versions, it computes a hash of each version and it filters out the duplicates.
 Finally, it builds a runner that embeds one version compressed (the source) and the others as compressed binary patches to the source.
-For instance, when building for the target `x86_64-pc-windows-msvc`, 37 different versions
+For instance, when building for the target `x86_64-pc-windows-msvc`, by default 37 different versions
 will be built, filtered, compressed, and merged into a single portable binary.
 
 When executed, the runner uncompresses and executes the version that matches the CPU features