Add documentation in the readme for adding proxy rules

[bsnider]

When using a proxy to authenticate on behalf of a user when accessing secured services, a proxy rule is needed. The code can be added after line 87 in main.js. Eg.

          urlUtils.addProxyRule({
            urlPrefix: "http://sampleserver6.arcgisonline.com",
            proxyUrl: "http://domain.com/DotNet/proxy.ashx"
          });

[jgravois]

do think this information belongs in the README.md or in the wiki? can you submit a pull request?