message: Update README.md
  enhanced example in readme
diff: @@ -19,6 +19,8 @@ Instances of DataFrame are not multithreaded safe. In other words, a single inst
  <B>Example Code</B>
  ```CPP
  +using namespace hmdf;
  +
  // Define a DataFrame with unsigned long index type
  typedef StdDataFrame<unsigned long> MyDataFrame;
  