Update Windows build instructions

Just a few suggested changes to reflect experiences over a few months of building on several Windows machines.
- The Posix tools comment doesn't seem to be applicable (anymore)? 
- Added further instructions when using pre-installed Git.
- Added the frequent issue of first-time build failures (random `node-gyp` errors), which don't reoccur on second try.
- Added a few extra words on opening a new issue, to clarify that appropriate details should be provided, if the issue hasn't been reported before.

Feel free to amend or suggest further changes.