Update build instructions: include mdbook plugins

Fixes #4018

README.md

@@ -33,6 +33,16 @@ rust-lang/rust uses in [this file][rust-mdbook]. To get it:
 $ cargo install mdbook --locked --version <version_num>
 ```
 
+The book also uses two mdbook plugins which are part of this repository. If you
+do not install them, you will see warnings when building and the output will not
+look right, but you *will* still be able to build the book. To use the plugins,
+you should run:
+
+```bash
+$ cargo install --locked --path packages/mdbook-trpl-listing
+$ cargo install --locked --path packages/mdbook-trpl-note
+```
+
 ## Building
 
 To build the book, type: