Updates information about Okio and Retrofit integration.

Since many Retrofit and OkHttp users may end trying to find about Retrofit and OkHttp integration in the OkHttp page/issues it would be good to have a reference about Okio and Retrofit at OkHttp readme and index.html
I don't think any of the changes of content in this file are correct.

 * OkHttp works with much more than Retrofit so it's strange to call it out specifically.
 * We have shims for HttpUrlConnection and Apache's HttpClient as well so only mentioning one without the other doesn't make sense. We can get around to adding information on them (probably on the Wiki) but I don't think we should just do one.