Added 'How to add header commands to compiletest'

Author: Xandkeeper

src/SUMMARY.md

@@ -3,6 +3,7 @@
 - [About this guide](./about-this-guide.md)
 - [How to build the compiler and run what you built](./how-to-build-and-run.md)
 - [Using the compiler testing framework](./running-tests.md)
+- [How to add new header commands to `compiletest`](./compiletest.md)
 - [Walkthrough: a typical contribution](./walkthrough.md)
 - [High-level overview of the compiler source](./high-level-overview.md)
 - [Queries: demand-driven compilation](./query.md)