Updating the upgrading doc

[donasaur]

Recently ran into a case where I had to upgrade the http-proxy version for an app, and that app was using a proxy table.

I'm not sure how many users are still using the 0.x.x version of http-proxy, but the added link (found from one of the GH issues) may encourage them to switch over to 1.x.x if they were using the old version due to its proxy table support and the activation energy to upgrade was too high.

Edit: There was some work involved in creating a proxy table to map paths to urls, disregarding the source's hostname, so I'm not sure if this would warrant creating a module (w/ source hostname support). Here's the relevant code if you also want to add it to the README:

    for (var pathPrefix in proxyTable) {
        if (proxyTable.hasOwnProperty(pathPrefix)) {
            if (path.indexOf(pathPrefix) === 0) {
                req.url = path.replace(pathPrefix, '');
                target = proxyTable[pathPrefix]; // this is the reverse proxy target
                break;
            }
        }
    }

[jcrugzz]

@donasaur yea a its been on my list to make a module to replicate some of the features that were lost in the 1.0.0 simplication so if you decide to write one, definitely send a PR to add it to the README. 👍.

Thanks for the reference here!