readme: credit @feliixx (#58)

Author: domodwyer

README.md

@@ -11,7 +11,7 @@ Further PR's (with tests) are welcome, but please maintain backwards compatibili
 * Fixes attempting to authenticate before every query ([details](https://github.com/go-mgo/mgo/issues/254))
 * Removes bulk update / delete batch size limitations ([details](https://github.com/go-mgo/mgo/issues/288))
 * Adds native support for `time.Duration` marshalling ([details](https://github.com/go-mgo/mgo/pull/373))
-* Reduce memory footprint / garbage collection pressure by reusing buffers ([details](https://github.com/go-mgo/mgo/pull/229))
+* Reduce memory footprint / garbage collection pressure by reusing buffers ([details](https://github.com/go-mgo/mgo/pull/229), [more](https://github.com/globalsign/mgo/pull/56))
 * Support majority read concerns ([details](https://github.com/globalsign/mgo/pull/2))
 * Improved connection handling ([details](https://github.com/globalsign/mgo/pull/5))
 * Hides SASL warnings ([details](https://github.com/globalsign/mgo/pull/7))