add javadoc about default compression threshold

[yev]

Hi Dustin,
I was struggling about 2 days to find out the issue with compressed image content which could not be served by nginx memcached anent (broken image).

I hope this little JavaDoc will help people to manage such situations

[daschl]

@yev thanks, we need to get this in through gerrit. Do you want to do this in your own or should I get it in and note you in the commit msg?

[yev]

@daschl you are welcome, you have gerrit outside the github? To make it simple, just push my patch with yours :)

[daschl]

@yev correct github is just a mirror. we use gerrit for code review. i'll push it through mine, cheers!

You can find more info here if you are curious: http://www.couchbase.com/wiki/display/couchbase/Contributing+Changes