Specify minimum browser requirements

[danielcompton]

I think the use of AbortController will set a fairly high minimum browser support level. It will mean dropping IE11 (which is fine as it doesn't have Fetch either), but it also requires fairly modern versions of other browsers like desktop Safari 12.1 (released in March 2019), and iOS Safari 11.3 (released in March 2018). Chrome and Firefox are evergreen browsers, so nearly everyone will be on very modern versions which have support for AbortController.

I don't think it's a problem, but it could be good to mention compatibility requirements in the README somewhere when you get closer to launching.

[superstructor]

Thanks for pointing this out @danielcompton, I'll add it once I get back to this project 👍