Return of Cygwin (also drop init.sh)

Git requires packages `libsasl2` and `ca-certificates` to work with submodules and https respectively.
Cygwin doesn't install these when installing git. This seems to be the cause of most issues for Cygwin users.
Clean up the readme again.
Merge pull request #241 from yenatch/cygwin

Bring back Cygwin instructions and get rid of init.sh