Add README

Author: nbdd0121

README.md

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+<!--
+Copyright lowRISC contributors.
+
+SPDX-License-Identifier: MIT OR Apache-2.0
+-->
+
+# parallel-cp
+
+This is a very simple tool that performs parallel local file copy.
+
+It's aimed at speeding up copying when one of the locally mounted file system is actually over network and therefore has high latency and `cp` or `rsync` is using tiny fraction of your network bandwidth or disk IO.
+
+Being a simple tool, it has minimal error handling -- use with care.
+
+## Usage
+
+`parallel-cp <SRC> <DST>`
+
+Both `<SRC>` and `<DST>` should be directories.
+They're not interpreted like `cp`: if you run `parallel-cp a b` and `b` is an existing directory, it does not copy `a` to `b/a`.
+This operates like `cp --no-target-directory`.
+Files nested under `<SRC>` with path `<SRC>/foo/bar` will be copied to `<DST>/foo/bar`.
+
+By default this tool has parallel equal to the number of CPU cores.
+For network file systems, this might be further increased to improve throughput.
+You can set `RAYON_NUM_THREADS` to override the default.