Document optimizations not yet implemented or out of scope for optimizer

In the project readme it says, “You can find the currently supported transformations [here](https://github.com/ampproject/amp-toolbox/blob/master/optimizer/lib/transformers).” But what are the _not_-supported transformations? If I incorporate the optimizer into my stack, what optimizations will be missing which are performed by the Google AMP Cache? There are currently [4 open issues](https://github.com/ampproject/amp-toolbox/issues) but there is no mention of the the image optimizations documented in [Google AMP Cache, Optimizations for Slow Networks, and the Need for Speed](https://developers.googleblog.com/2017/01/google-amp-cache-amp-lite-and-need-for.html). The image optimizations are likewise not mentioned in the [AMP Cache Guidelines](https://github.com/ampproject/amphtml/blob/master/spec/amp-cache-guidelines.md) nor [AMP cache modifications best practices](https://github.com/ampproject/amphtml/blob/master/spec/amp-cache-modifications.md). There feels like a fragmentation of all the optimizations which _may_ be performed by Google's AMP Cache. Having an exhaustive list of what exactly is done would be helpful for others who want to understand how their content is modified and for those who want to stand up their own caches.

I'm looking to identify what could be implemented at the hosting layer for WordPress installs to get (close to) the same speed benefits for browsers on origin that browsers do on an AMP cache. In addition to documenting which optimizations are _not_ included in the amp-toolbox, having everything documented would be key to getting documentation for an “AMP Cache for Hosts”: providing guidelines for hosts to be able to integrate AMP caching optimizations into their own caching infrastructure so that users visiting origin directly on native AMP sites get almost all the same speed benefits as users accessing a site via an AMP cache

Start with the README’s supported-transformations link and inspect optimizer/lib/transformers. Compare those transformations with the linked Google AMP Cache and AMP specification documents, including the image optimizations. Done means documenting an exhaustive, current list of supported, missing, and out-of-scope cache optimizations, including guidance relevant to host-level caching.

Written by the indexing model from the issue text.