Add Visual Studio Code documentation

Adds documentation about installation and usage of Visual
Studio Code as an editor for Chromium. The documentation
is to be referenced from the Chromium developers web page,
https://www.chromium.org/developers, as a sibling of Atom,
Sublime Text etc.

BUG=None
TEST=None

Review-Url: https://codereview.chromium.org/2710503003
Cr-Commit-Position: refs/heads/master@{#463964}