Add link to "released" version ZIP files

[bsvensson]

Adding a link to https://github.com/Esri/resource-proxy/releases should help git-newbies quickly find the right thing to download.

Current instructions in main README.md only say:

• Download and unzip the .zip file or clone the repository.

[esoekianto]

@bsvensson @kellyhutchins
can we point the link from javascript api (or flex or silverlight) developer page to point to the release download instead of main page. this is to make sure that user will download the working latest release copy instead of master branch copy?