Update SSR README
Update SSR README

Make it less about ssr and more about general optimizations.

Change-Id: Id51632d1f07b333f07b0ad8bdf67e71120639265

ampproject/amp-toolbox,ampproject/amp-toolbox,ampproject/amp-toolbox,google/amp-toolbox,google/amp-toolbox,google/amp-toolbox,ampproject/amp-toolbox