Add reference to Yahoo code in NOTICE file

Some files were taken from ycsb so we need to say that in NOTICE file:

./benchmarks/src/main/java/org/apache/omid/benchmarks/utils/Generator.java
./benchmarks/src/main/java/org/apache/omid/benchmarks/utils/IntegerGenerator.java
./benchmarks/src/main/java/org/apache/omid/benchmarks/utils/ScrambledZipfianGenerator.java
./benchmarks/src/main/java/org/apache/omid/benchmarks/utils/ZipfianGenerator.java