Substitute usage information into the documentation

This will simplify keeping documentation in sync with the code.

docs/README.md

@@ -1,3 +1,10 @@
 This is an [mdbook](https://rust-lang.github.io/mdBook/index.html) book.
 Minor modifications can be made without installing the mdbook tool, as the content is simple Markdown.
 Changes are verified on pull requests.
+
+To build the docs locally, you will need to build `usage-docs`:
+
+```
+cargo build -p taskchampion-cli --feature usage-docs --bin usage-docs
+mdbook build docs/
+```