Update Windows build instruction

[martincostello]

The Windows instruction says to run build.ps1, but there's no such file.

[dagood]

Hi @martincostello, Windows isn't currently supported in this repo: #1190. You'll see that even build.cmd turns around and tries to call the non-existent powershell script:

source-build/build.cmd

Line 2 in 71e0ada

powershell -NoProfile -NoLogo -Command "& \"%~dp0build.ps1\" %*; exit $LastExitCode;"

Also, our main branch is release/3.1, if you'd like to help us update the readme there.

[martincostello]

Fair enough - I only did a cursory read of the repo after seeing it in the 5.0 preview 4 release blog post. If it doesn't work at all for Windows, I'll just leave it.

[dagood]

Yeah, wish we had some time to set aside to make sure our docs and contrib guidance was clear before that link. This repo is quite unusual compared to the ones that make up the main components. Sorry for the confusion.