Add a chapter describing mdbook-spec

Author: ehuss

reference-dev-guide/src/SUMMARY.md

@@ -5,7 +5,7 @@
 - [Publishing process](publishing.md)
 - [Reference tooling]()
     - [Building the Reference](tooling/building.md)
-    - [mdbook-spec]()
+    - [mdbook-spec](tooling/mdbook-spec.md)
 - [Tests]()
 - [Formatting]()
     - [Markdown]()

reference-dev-guide/src/tooling/mdbook-spec.md

@@ -0,0 +1,39 @@
+# mdbook-spec
+
+[`mdbook-spec`] is an mdBook preprocessor that adds features to the Reference. It provides:
+
+- Parsing and generation of [grammar diagrams].
+    - [Automatic grammar production links].
+    - Generation of the [grammar summary appendix].
+- [Automatic standard library links].
+- Handling of [rule names].
+    - Validation of the names.
+    - Converting rule names to links.
+    - [Automatic rule link references].
+    - Generation of [links to rule tests].
+    - Generation of the [test summary].
+- Support for [admonitions].
+
+## Environment variables
+
+There are a few environment variables that `mdbook-spec` uses, described in the [building chapter]:
+
+- [`SPEC_RELATIVE`] --- Can be set to link external books to the live site.
+- [`SPEC_DENY_WARNINGS`] --- Whether warnings should be treated as errors.
+- [`SPEC_RUST_ROOT`] --- The path to a checkout of the [`rust-lang/rust`] GitHub repository. This is used for test linking.
+
+[`mdbook-spec`]: https://github.com/rust-lang/reference/tree/master/mdbook-spec
+[`rust-lang/rust`]: https://github.com/rust-lang/rust
+[`SPEC_DENY_WARNINGS`]: building.md#SPEC_DENY_WARNINGS
+[`SPEC_RELATIVE`]: building.md#SPEC_RELATIVE
+[`SPEC_RUST_ROOT`]: building.md#SPEC_RUST_ROOT
+[admonitions]: ../formatting/admonitions.md
+[Automatic grammar production links]: ../grammar.md#automatic-linking
+[Automatic rule link references]: ../links.md#rule-links
+[Automatic standard library links]: ../links.md#standard-library-links
+[Building chapter]: building.md
+[grammar diagrams]: ../grammar.md
+[grammar summary appendix]: https://doc.rust-lang.org/nightly/reference/grammar.html
+[links to rule tests]: ../rules/test-annotations.md
+[rule names]: ../rules/index.md
+[test summary]: https://doc.rust-lang.org/nightly/reference/test-summary.html