doc: add g3doc sitemap to toplevel readme

BUG=None

No-Try: true
Change-Id: I18d0ff22089271f364651955f9718f7c603fca48
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/225542
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34538}
diff --git a/README.md b/README.md
index 37694d9..332efcc 100644
--- a/README.md
+++ b/README.md
@@ -27,5 +27,6 @@
  * [Coding style guide](g3doc/style-guide.md)
  * [Code of conduct](CODE_OF_CONDUCT.md)
  * [Reporting bugs](docs/bug-reporting.md)
+ * [Documentation](g3doc/sitemap.md)
 
 [native-dev]: https://webrtc.googlesource.com/src/+/main/docs/native-code/index.md