IE 11 Support about async-sema

[HaveSpacesuit]

I needed to have Babel do it's thing to the package for it to work in IE. By default, we were excluding node_modules from compilation. I update our webpack config to something like this, which solved the issue.

exclude: function(modulePath) {
            return (
              /node_modules/.test(modulePath) && !/async-sema/.test(modulePath)
            );
          },

[OlliV]

You could try changing this to ES5
https://github.com/zeit/async-sema/blob/95cf8e48e990f1a866fd234f34b6f60c97afaeb2/tsconfig.json#L15

We once merged this PR but it made the package a lot bigger than we liked.

[OlliV]

might be worth of adding this to the readme just in case.