Move image used in top level README to top level

This Greenplum logo png is used in the top level README of the
repository. Instead of having the image live a few layers deep in the
gpAux/releng directory (which we will eventually be removing), have the
image be placed in close proximity to the README.md itself.
Authored-by: NKris Macoskey <kmacoskey@pivotal.io>

README.md

@@ -3,7 +3,7 @@
 
 ----------------------------------------------------------------------
 
-![Greenplum](/gpAux/releng/images/logo-greenplum.png)
+![Greenplum](logo-greenplum.png)
 
 The Greenplum Database (GPDB) is an advanced, fully featured, open
 source data warehouse. It provides powerful and rapid analytics on