README.md: fix cmake error "No build type selected"

[wenju-he]

Fix following error:

CMake Error at CMakeLists.txt:97 (message):


  No build type selected.  You need to pass -DCMAKE_BUILD_TYPE=<type> in
  order to configure LLVM.

  Available options are:

    * -DCMAKE_BUILD_TYPE=Release - For an optimized build with no assertions or debug info.
    * -DCMAKE_BUILD_TYPE=Debug - For an unoptimized build with assertions and debug info.
    * -DCMAKE_BUILD_TYPE=RelWithDebInfo - For an optimized build with no assertions but with debug info.
    * -DCMAKE_BUILD_TYPE=MinSizeRel - For a build optimized for size instead of speed.

  Learn more about these options in our documentation at
  https://llvm.org/docs/CMake.html#cmake-build-type



-- Configuring incomplete, errors occurred!