Add files.watcherExlude to vscode.md

I had the issue that vscode didn't notice file changes e.g.
by git cl format. Excluding out/ and third_party/ with
files.watcherExclude seems to help.

Files per folder:
10934 v8
14436 native_client
17478 data
26470 build
37676 chrome
389528 third_party
426532 out

BUG=

Review-Url: https://codereview.chromium.org/2847723002
Cr-Commit-Position: refs/heads/master@{#467668}
1 file changed