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 +1,5 @@
+# Reference tooling
+
+The Reference uses [mdBook] to convert its source files from Markdown to HTML. See [Building the Reference](building.md) for more information. Additionally, there is an mdBook extension called [mdbook-spec](mdbook-spec.md) 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](../tests.md).