Fix docs around Windows use

[srl295]

Comments such as #195 (comment) seem to indicate that the README.md is broken as regards Windows use. Can you add a pointer around this point to the right path to take? Otherwise Windows use isn't mentioned after that point. But "Easy" is mentioned a couple of times! Perhaps something like:
- Note: this won't work on Windows, see here
Also if the commands/docs are broken, shouldn't the issue stay open?

[fehguy]

With the move from scala in codegen, this issue is now resolved. You can run with maven or with the executable jar under swagger-codegen-distribution.

[srl295]

@fehguy thanks!