Document windows toolchain update process

[codebytere]

Chromium invariably updates the toolchain package needed to build Chromium on Windows, which we then also need to update in Electron. We should document the steps to do that.

See https://chromium-review.googlesource.com/c/chromium/src/+/4072083 for an example

cc @jkleinsc

[dsanders11]

Can we also document how the Xcode zip files are created? 🙂