#89 update README.md

### To run this FAISS benchmark, please follow these steps:

#### Step 1:

Download the HDF5 source from:

  https://support.hdfgroup.org/ftp/HDF5/releases/

and build/install to "/usr/local/hdf5".

#### Step 2:

Download HDF5 data files from:

  https://github.com/erikbern/ann-benchmarks

#### Step 3:

Update 'milvus/core/src/index/unittest/CMakeLists.txt',

uncomment "#add_subdirectory(faiss_benchmark)".

#### Step 4:

Build Milvus with unittest enabled: "./build.sh -t Release -u",

binary 'test_faiss_benchmark' will be generated.

#### Step 5:

Put HDF5 data files into the same directory with binary 'test_faiss_benchmark'.

#### Step 6:

Run test binary 'test_faiss_benchmark'.

To run this FAISS benchmark, please follow these steps:

1. Download the HDF5 from:

     https://support.hdfgroup.org/ftp/HDF5/releases/

   and install to /usr/local/hdf5

2. Download HDF5 data files from:

     https://github.com/erikbern/ann-benchmarks

3. Put HDF5 data files into the same directory with test binary

4. Run the test binary