README: Update the Caveats section

[dylanahsmith]

The Caveats section of the README contained some stale information, which I removed. For example,

If an after_commit fails before the cache expiry after_commit the cache will not be expired and you will be left with stale data.

is no longer true after #471.

Since everything is being marshalled and unmarshalled from Memcached changing Ruby or Rails versions could mean your objects cannot be unmarshalled from Memcached.

is no longer true after #242

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I've also turned the brief descriptions of ways in which cache invalidations can be lost into a list. This should help a reader looking for the potential source of lost cache invalidations as well as for us to easily add to. I've also added 2 additional ways cache invalidations can be lost to the list that didn't seem to be documented, one that can occur during cache flushing and another that can occur during cache namespace changes.

I also wanted the caveats to explicitly discourage the use of Identity Cache for write requests.

Lastly, I wanted to document the problem with queuing background jobs with ids for uncommitted database records and how can lead to nil being cached for these records.