rust: move hgcli's README out of the way
My understanding is that it's not meant to be used in the current
form.
Differential Revision: https://phab.mercurial-scm.org/D7951
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Mercurial Rust Code
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This directory contains various Rust code for the Mercurial project.
The top-level ``Cargo.toml`` file defines a workspace containing
all primary Mercurial crates.
Building
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To build the Rust components::
$ cargo build
If you prefer a non-debug / release configuration::
$ cargo build --release