distance metric parameter: Not Mentioned in README

[JigaoLuo]

DiskANN/tests/build_disk_index.cpp

Line 33 in bc39cd0

if (std::string(argv[2]) == std::string("mips"))

DiskANN/tests/search_disk_index.cpp

Line 62 in bc39cd0

if (std::string(argv[ctr]) == std::string("mips"))

DiskANN/tests/build_memory_index.cpp

Line 60 in bc39cd0

if (std::string(argv[ctr]) == std::string("mips"))

DiskANN/tests/search_memory_index.cpp

Line 33 in bc39cd0

if (std::string(argv[ctr]) == std::string("mips"))

This distance metric parameter is not mentioned in README.
README needs to be updated.