readme needs updating

• Note1: The current Debian stable versions are Jessie and Stretch. Stretch version does not work, as when you try to login in the dashboard you get a CSRF error.
  The problem with Stretch is documented in the lava-server-doc package - recent versions of Django in Stretch insist of security settings which prevent logging in to http://localhost or other http URLs without settings being changed in {{/etc/lava-server/settings.conf}}
  https://validation.linaro.org/static/docs/v2/pipeline-debug.html#using-localhost-or-non-https-instance-url

Please also update your readme based on the documentation which shows how to get updated versions of LAVA from stretch-backports and the LAVA repositories.

Please also remove the obsolete section on using Windows - there is no support for doing this. No-one should be using LAVA other than using Debian packages.

[ader1990]

Thanks for the info. I will update the Readme accordingly.