MINIFICPP-1200: Add MiNiFi Logo to README
MINIFICPP-1200: Add MiNiFi Logo to README

Adds MiNiFi Logo to the README.md.
Also includes an image hyperlink to the MiNiFi documentation homepage: https://nifi.apache.org/minifi/

Signed-off-by: Arpad Boda <aboda@apache.org>

This closes #764

jdye64/nifi-minifi-cpp,jdye64/nifi-minifi-cpp,dtrodrigues/nifi-minifi-cpp,dtrodrigues/nifi-minifi-cpp,achristianson/nifi-minifi-cpp,achristianson/nifi-minifi-cpp,dtrodrigues/nifi-minifi-cpp,jdye64/nifi-minifi-cpp,dtrodrigues/nifi-minifi-cpp,apiri/nifi-minifi-cpp,jdye64/nifi-minifi-cpp,achristianson/nifi-minifi-cpp,apiri/nifi-minifi-cpp,apiri/nifi-minifi-cpp,achristianson/nifi-minifi-cpp,apiri/nifi-minifi-cpp,jdye64/nifi-minifi-cpp,dtrodrigues/nifi-minifi-cpp,apiri/nifi-minifi-cpp