Add supported OS Versions to installation docs

[jennifer-shehane]

The Cypress Test Runner (desktop app) supports whatever OS versions Electron supports. This should be specified in the installation docs. Original Electron document

• Mac: macOS 10.9+
• Windows: Windows 7+ (coming soon)
• Linux: Ubuntu 12.04+, Fedora 21, Debian 8

[arnazmursidp]

Hi, i want to contribute with this branch, i'm a first timer, thank you!