Add browser support info to README

[DavidFrahm]

Would be helpful to users considering this library to have readily-available browser support information.

Examples:

• Simply mention that any browser that supports HTML File API (maybe also just link to caniuse.com?)
• I see there are tests, so could simply list what browsers those tests are run against (and whether that's a manual or automated process)
• If none of the above really fit, then add a manually maintained list or helpful information.