Add an introduction to the tooling sub-chapters

Author: ehuss

reference-dev-guide/src/SUMMARY.md

@@ -3,7 +3,7 @@
 - [Introduction](introduction.md)
 - [Contribution process]()
 - [Publishing process](publishing.md)
-- [Reference tooling]()
+- [Reference tooling](tooling/index.md)
     - [Building the Reference](tooling/building.md)
     - [mdbook-spec](tooling/mdbook-spec.md)
 - [Tests](tests.md)

reference-dev-guide/src/tooling/index.md

@@ -0,0 +1,10 @@
+# Reference tooling
+
+The Reference uses [mdBook] to convert its source files from Markdown to HTML. See [Building the Reference] for more information. Additionally, there is an mdBook extension called [`mdbook-spec`] which adds a variety of custom features used by the Reference.
+
+In addition to the book tooling, there is a variety of testing tools documented in the [tests chapter].
+
+[`mdbook-spec`]: mdbook-spec.md
+[Building the Reference]: building.md
+[mdbook]: https://rust-lang.github.io/mdBook/
+[tests chapter]: ../tests.md