Add README.md to ep-engine
Add README.md to ep-engine

Document some basics of threading and synchronisation.

Change-Id: I2d462df481bba73784de0d177b28f9aabe257d15
Reviewed-on: http://review.couchbase.org/56192
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>
Reviewed-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>

couchbase/ep-engine,daverigby/ep-engine,daverigby/kv_engine,owendCB/ep-engine,teligent-ru/ep-engine,hisundar/ep-engine,daverigby/kv_engine,daverigby/kv_engine,teligent-ru/ep-engine,daverigby/ep-engine,owendCB/ep-engine,membase/ep-engine,hisundar/ep-engine,couchbase/ep-engine,couchbase/ep-engine,owendCB/ep-engine,teligent-ru/ep-engine,daverigby/ep-engine,hisundar/ep-engine,daverigby/ep-engine,membase/ep-engine,hisundar/ep-engine,teligent-ru/ep-engine,daverigby/kv_engine,owendCB/ep-engine,membase/ep-engine,couchbase/ep-engine,membase/ep-engine