Update README to recomend Windows 10
Update README to recomend Windows 10

Summary:
Why?
- Apparently there is a performance bug on Windows 7: https://github.com/facebook/flow/issues/6517
- I wonder if other Windows-specific issues are on older versions of Windows. Not unlikely.
- We develop on Windows 10

Reviewed By: gabelevi

Differential Revision: D8813140

fbshipit-source-id: 84a0a7eaf590559fd3e37f05102d1650f0be733b

gabelevi/flow,facebook/flow,popham/flow,popham/flow,mroch/flow,claudiopro/flow,claudiopro/flow,claudiopro/flow,facebook/flow,nmote/flow,mroch/flow,AgentME/flow,samwgoldman/flow,mroch/flow,facebook/flow,facebook/flow,jamesgpearce/flow,gabelevi/flow,jamesgpearce/flow,AgentME/flow,samwgoldman/flow,facebook/flow,samwgoldman/flow,nmote/flow,popham/flow,jamesgpearce/flow,popham/flow,JonathanUsername/flow,samwgoldman/flow,samwgoldman/flow,AgentME/flow,jamesgpearce/flow,mroch/flow,mroch/flow,samwgoldman/flow,jamesgpearce/flow,JonathanUsername/flow,gabelevi/flow,nmote/flow,JonathanUsername/flow,gabelevi/flow,AgentME/flow,gabelevi/flow,nmote/flow,mroch/flow,jamesgpearce/flow,gabelevi/flow,claudiopro/flow,JonathanUsername/flow,nmote/flow,popham/flow,JonathanUsername/flow,gabelevi/flow,mroch/flow,AgentME/flow,facebook/flow,samwgoldman/flow,popham/flow,facebook/flow,claudiopro/flow,nmote/flow,nmote/flow,JonathanUsername/flow,popham/flow,JonathanUsername/flow,jamesgpearce/flow