build instruction & link to the sigmod paper

Hi Rusty,

Could you document how to compile bLSM?

Also, it would be nice to add a link to the paper whenever it becomes available.

I'm planning to add bLSM support for mapkeeper. I think many people will be interested in comparing it to bdbj and leveldb.

https://github.com/m1ch1/mapkeeper/issues/15

Thanks!
--Michi