docs: mention isolate --no-build for swarming

Thanks thakis@ :)

CC=thakis@chromium.org
R=agable@chromium.org

Bug: None
Change-Id: Ida6a7bcf5f59178d39546eb9f1c0d2c6810366cd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1796202Reviewed-by: Aaron Gable <agable@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695278}

docs/workflow/debugging-with-swarming.md

@@ -64,7 +64,12 @@ $ tools/mb/mb.py isolate //$outdir $target
 ```
 
 This will produce some files in $outdir. The most pertinent two are
-`$outdir/$target.isolate` and $outdir/target.isolated`.
+`$outdir/$target.isolate` and `$outdir/target.isolated`. If you've already built
+$target, you can save some CPU time and run `tools/mb/mb.py` with `--no-build`:
+
+```
+$ tools/mb/mb.py isolate --no-build //$outdir $target
+```
 
 Support for building an isolate using swarming, which would allow you to build
 for a platform you can't build for locally, does not yet exist.