Add statement of supported browsers

[jryans]

The README or some page linked from it should list our supported browsers.

[jryans]

Ah, I guess we sort of have something in the React SDK developer guide. We should lift it up to Riot Web, and also perhaps be more explicit with versions, such as "last 2 stable releases", "latest stable version only", or similar.

[jryans]

There is also CompatibilityPage, which should match whatever statement we make in our docs.

[turt2live]

Official ruling is:

Firefox Chrome Safari, latest versions.

Edge is supported if it works, but not actively maintained.