Document SPEC_DENY_WARNINGS

Author: ehuss

README.md

@@ -77,3 +77,7 @@ SPEC_RELATIVE=0 mdbook serve --open
 The `SPEC_RELATIVE=0` environment variable makes links to the standard library go to <https://doc.rust-lang.org/> instead of being relative, which is useful when viewing locally since you normally don't have a copy of the standard library.
 
 The published site at <https://doc.rust-lang.org/reference/> (or local docs using `rustup doc`) does not set this, which means it will use relative links which supports offline viewing and links to the correct version (for example, links in <https://doc.rust-lang.org/1.81.0/reference/> will stay within the 1.81.0 directory).
+
+### `SPEC_DENY_WARNINGS`
+
+The `SPEC_DENY_WARNINGS=1` environment variable will turn all warnings generated by `mdbook-spec` to errors. This is used in CI to ensure that there aren't any problems with the book content.