README: Clarify build instructions, as we need to patch clang
README: Clarify build instructions, as we need to patch clang

Signed-off-by: Valentin Ilie <b44f05d43a7ee70b9a0a8271f9e9e9eea220c025@intel.com>

ucisysarch/opencvjs,sajjadt/opencvjs,sajjadt/opencvjs,sajjadt/opencvjs,ucisysarch/opencvjs,ucisysarch/opencvjs,sajjadt/opencvjs,ucisysarch/opencvjs