Add a README for WebCache

R=ojan@chromium.org

Change-Id: Idbbd5e7bed12d41c8b53dd99233386d502bd8da3
Reviewed-on: https://chromium-review.googlesource.com/778323Reviewed-by: Ojan Vafai <ojan@chromium.org>
Commit-Queue: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517868}

components/web_cache/README.md

+The WebCache component aims to coordinate the in-memory cache resource
+usage of all renderers to not exceed a certain threshold while ensuring
+that the active renderer has the most memory available for caching.