ZOOKEEPER-3120: add NetBeans nbproject directory to .gitignore

Author: Enrico Olivelli <eolivelli@apache.org>

Reviewers: fangmin@apache.org, andor@apache.org

Closes #598 from eolivelli/fix/netbeans-nbproject

.gitignore

@@ -33,6 +33,7 @@ out/
 # NetBeans
 *~.nib
 nbbuild/
+nbproject/
 dist/
 nbdist/
 .ndb-gradle/