How to Contribute has confusing build instructions

[dnicolson]

Under the Build section there are seemingly VS Code commands in place of where command line commands would be. It appears as though this was introduced in 659f67e when yarn watch was removed and replaced by a VS Code command.

The previous iteration (https://github.com/microsoft/vscode-wiki/blob/2e2d6b0/How-to-Contribute.md#build) is much clearer and the VS Code build option is secondary to the command line option (it also might be a little unintuitive to use VS Code to make a development build of VS Code).

[joaomoreno]

Improved: https://github.com/microsoft/vscode/wiki/How-to-Contribute#build