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.
corrected grammar
typo
Merge pull request #3022 from batjko/patch-1

Update Windows build instructions