Questions.md: add link to codesearch

I could have answered my own question recently about a mysterious filename
on a problematic LUCI builder if I had known how to search the Go respositories
rather than just the compiler and runtime.

It seems overkill to create an issue for a two-line change, but if we need to
coordinate wider discussion about the correct wiki page or whatever, will do so.

Change-Id: If02fce333e48d1d073ce2ddbadd080a76d74975c
Reviewed-on: https://go-review.googlesource.com/c/wiki/+/688995
Reviewed-by: Michael Pratt <[email protected]>
Auto-Submit: Michael Pratt <[email protected]>

Author: n2vi

Questions.md

@@ -25,3 +25,6 @@ For asking questions, see:
 If we closed your issue as a question with a link to this wiki, we apologize. Please ask the question on one of the above forums.
 
 Please do not write your question on a wiki page.  Please use one of the above forums.
+
+As with any open source project, sometimes you can answer your own question by reading the source, if you know where to start.
+The search bar at [cs.opensource.google/go](https://cs.opensource.google/go) can help.