Update README.md

Heroku is not detecting calibre library path.

Error was: 
Docverter::APIError: Docverter API Error: command error: ebook-convert: error while loading shared libraries: libcalibre-launcher.so: cannot open shared object file: No such file or directory

was able to resolve by ` heroku config:add LD_LIBRARY_PATH=/app/calibre/lib`