Mention max cookie size in docs

Closes #2 - mentioning the max cookie size.

Sensible idea to draw people's attention to the fact that cookies can't be too big.

Opinion: should we have the code work out the size of the cookie when flushing, and if it's > 4KB, throw a warning to the logs that it's likely to be rejected by browsers, to help developers debugging issues?  Or is this sufficient?