Add a warning in the README and code about the fallback fetcher

[dylanahsmith]

As mentioned in https://github.com/Shopify/identity_cache/issues/268#issuecomment-250588856

It looks like we are missing a warning about this in the README and the code. The README even uses ActiveSupport::Cache::MemCacheStore as an example for configuring identity cache, which doesn't .respond_to?(:cas) or .respond_to?(:cas_multi) so it will use the fallback fetcher. Having a warning if the fallback fetcher is used would make this mis-configuration easier to catch and make it easier to discover this problem when upgrading.

Otherwise, users will run into issues like https://github.com/Shopify/identity_cache/issues/268 about cache corruption, even though this is a known problem.

We might want to mention other pitfalls about caching like how memcache clients normally fallback to use a different server for a cache key, which can cause an invalidation to go to the wrong server without noticing. I remember that we configured memcache to not do this. It is still possible that cache invalidation could get missed because of connection errors with sending invalidation operations to memcached, but that could at least be reduced by not falling back to another server.