Call out #Caveats higher up in readme

https://github.com/github/fetch/issues/787

README.md

@@ -31,6 +31,8 @@ replacement for most uses of XMLHttpRequest in traditional web applications.
 * If you believe you found a bug with how `fetch` behaves in your browser,
   please **don't open an issue in this repository** unless you are testing in
   an old version of a browser that doesn't support `window.fetch` natively.
+  Make sure you read this _entire_ readme, especially the [Caveats](#caveats)
+  section, as there's probably a known work-around for an issue you've found.
   This project is a _polyfill_, and since all modern browsers now implement the
   `fetch` function natively, **no code from this project** actually takes any
   effect there. See [Browser support](#browser-support) for detailed