specifying rustc version requirement in pcli installation docs

docs/guide/src/pcli/install.md

@@ -2,11 +2,13 @@
 
 ### Installing the Rust toolchain
 
-This requires that you install a recent stable version
+This requires that you install a recent (>= 1.7.3) stable version
 of the Rust compiler, installation instructions for which you can find
 [here](https://www.rust-lang.org/learn/get-started). Don't forget to reload your shell so that
 `cargo` is available in your `\$PATH`!
 
+You can verify the rust compiler version by running `rustc --version` which should indicate version 1.7.3 or later.
+
 `pcli` requires `rustfmt` as part of the build process --- depending on your
 OS/install method for Rust, you may have to install that separately.