Update CMakeLists.txt

Added  some notes for building the boost.
Note: In `src/mapreduce-lite/CMakeLists.txt`, we referred to multithreading version of `boost` libraries, e.g, `libboost_thread-mt`.  However, depends on how you install `boost`, there may not have multithreading version of libraries on your development system. You might need to check by issuing commands like `ls <BOOST_DIR>/lib/libboost*-mt`.  Depending on your check result, you might want to edit `src/mapreduce-lite/CMakeLists.txt` to use the single-threaded version instead.