Enhance main README.md
Enhance main README.md

hifive-snu/hifive-test-explorer,hifive/hifive-test-explorer,hifive-snu/hifive-test-explorer,hifive/hifive-test-explorer,hifive/hifive-test-explorer,hifive-snu/hifive-test-explorer,hifive/hifive-pitalium-explorer,hifive/hifive-pitalium-explorer,hifive/hifive-pitalium-explorer,hifive/hifive-pitalium-explorer,hifive/hifive-test-explorer,hifive-snu/hifive-test-explorer