xtask: add README.md

Add documentation in a README.md file.

Signed-off-by: Manos Pitsidianakis <[email protected]>

Author: epilys

xtask/README.md

@@ -0,0 +1,22 @@
+# `xtask` - Run tasks with `cargo`
+
+This binary crate provides support for running useful tasks with `cargo xtask <..>`.
+
+## `mangen`
+
+The `mangen` task which is enabled by the `mangen` cargo feature, builds ROFF manual pages for binary crates in this repository. It uses the [`clap_mangen`](https://crates.io/crates/clap_mangen) crate to generate ROFF from the crate's argument types which implement the `clap::CommandFactory` trait, through the `clap::Parser` derive macro.
+
+```session
+$ cargo xtask mangen
+    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.04s
+     Running `target/debug/xtask mangen`
+Generated the following manual pages:
+/path/to/rust-vmm/vhost-device/target/dist/man/vhost-device-sound.1
+/path/to/rust-vmm/vhost-device/target/dist/man/vhost-device-scmi.1
+```
+
+The following crates have manual pages built by default:
+
+- [`vhost-device-sound`](../vhost-device-sound), enabled by the default feature `vhost-device-sound`.
+  - It can further be fine-tuned with the features `vhost-device-sound-pipewire` and `vhost-device-sound-alsa`.
+- [`vhost-device-scmi`](../vhost-device-scmi), enabled by the default feature `vhost-device-scmi`.