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
This what does not belong here? 
1) The gradle reference? 
2) The integration section? 
3) All of it?

1 - I thought the gradle reference would belong to that place because there is alredy the maven reference there.

2 - About the integration section...
Yes, it was mentioned in the retrofit page, I did the documentation pull request there also.
I thought the info at Retrofit page would be a good info to who is beginning use Retrofit and OkHttp, but for those who are just updating from previous versions and see the stacktrack of the missing OkUrlFactory ( #879 )  and the OkHttp-UrlConnection it would be natural to come to Okhttp repo/page and try to find any documentation.

4 - If all of the pull request does not make sense to you, please close it.

Note: I am not very familiar with OkHttp-UrlConnection module (and the others modules at this same repo ) but it would be good to have some sort of section in the web page referencing it. Like, when OkHttp would need OkHttp-UrlConnection to work (like in the Retrofit case)?