Move the xtask binary to a library

So you can use it in your own projects with minimal copying and pasting.

nih_plug_xtask/README.md

@@ -0,0 +1,45 @@
+# NIH-plug: bundler and other utilities
+
+This is NIH-plug's `cargo xtask` command, but as a library. This way you can use
+it in your own projects without forking this repo or copying the binary into
+your own repo. This is necessary until Cargo supports [running binaries from
+dependencies directly](https://github.com/rust-lang/rfcs/pull/3168).
+
+To use this, add an `xtask` binary to your project using `cargo new --bin xtask`
+and add it to the Cargo workspace in your repository's main `Cargo.toml` file:
+
+```toml
+# Cargo.toml
+
+[workspace]
+members = ["xtask"]
+```
+
+Then add `nih_plug_xtask` to your new xtask package's dependencies:
+
+```toml
+# xtask/Cargo.toml
+
+[dependencies]
+nih_plug_xtask = { git = "https://github.com/robbert-vdh/nih-plug" }
+```
+
+Call `nih_plug_xtask`'s main function own xtask binary:
+
+```rust
+# xtask/src/main.rs
+
+fn main() -> nih_plug_xtask::Result<()> {
+    nih_plug_xtask::main()
+}
+```
+
+And finally create a `.cargo/config` file in your repository and add a Cargo
+alias so you can invoke the binary with `cargo xtask`:
+
+```toml
+# .cargo/config
+
+[alias]
+xtask = "run --package xtask --release --"
+```