doc xtask

xtask/README.md

+# Xtask
+
+We choose [`xtask`](https://github.com/matklad/cargo-xtask/) to run Rust scripts using `cargo`. To build these scripts, just run:
+
+```bash
+cargo xtask # this will build the scripts and show the available commands
+```
+
+These scripts mainly deal with runtime operations.
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