Clarify that this package doesn't work on the web in README

[filiph]

Since this package uses Isolate.spawn, it won't work for web apps (unless those apps use it in a non-DOM isolate). dart-lang/sdk#17649

This should be mentioned prominently in the readme, lest we frustrate people who try this package for this very reason.

I can prepare the PR if you agree with this.

[filiph]

I just realized it's not just that, but also the fact that IsolateRunner.run() takes a closure and Isolates on the web can't serialize closures.

Since IsolateRunner doesn't provide any function to send/receive values (like with the underlying isolate receivePort/sendPort architecture), it's not useful on the web even if the developer chooses to use it manually (using IsolateRunner() constructor instead of IsolateRunner.spawn()).

[lrhn]

Adding this to the readme.