add documentation on modifying the tool list

Author: hectorsector

contributing/content-markup-reference.md

@@ -128,7 +128,7 @@ You can define a default platform in the frontmatter. For more information, see
 
 ## Tool tags
 
-We occasionally need to write documentation for different tools (GitHub UI, GitHub CLI, GitHub Desktop, cURL, Codespaces, VS Code, GitHub Enterprise Importer CLI, GraphQL API). Each tool may require a different set of instructions. We use tool tags to demarcate information for each tool.
+We occasionally need to write documentation for different tools (GitHub UI, GitHub CLI, GitHub Desktop, cURL, Codespaces, VS Code, GitHub Enterprise Importer CLI, GraphQL API). Each tool may require a different set of instructions. We use tool tags to demarcate information for each tool. To modify the list of possible tools, edit [`lib/all-tools.js`](../lib/all-tools.js).
 
 ### Usage