add instruction for generating toc

Author: Feiyang1

repo-scripts/api-documenter/README.md

@@ -3,3 +3,12 @@
 It is a fork of [API Documenter](https://github.com/microsoft/rushstack/tree/master/apps/api-documenter)
 It reads the *.api.json data files produced by [API Extractor](https://api-extractor.com/),
 and then generates files in [Markdown](https://en.wikipedia.org/wiki/Markdown) format suitable for displaying in Firebase Devsite.
+
+## Generate toc for Firebase devsite
+`api-documenter-fire toc -i temp -p "/docs/reference/js/v9"`
+
+`-i` and `-p` are required parameters.
+Use `-i` to specify the folder that contains api.json files.
+Use `-p` to specify the g3 path that contains the reference docs.
+
+By default, the command will create `toc.yaml` in folder `/toc`. To change the output folder, use the flag `-o`.