#89 add README.txt

Former-commit-id: a84501ce6c2c94275819ace9e7d7a4afc14fbeca

core/src/index/unittest/faiss_benchmark/README.txt

+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
+