Added readme for setting up pandoc path on windows

I installed the plugin on Windows today and I found the plugin cannot found my pandoc.exe on my computer. To solve the problem, I need to do either:
A. change the 'pandoc' to 'pandoc.exe' in the python source code
or
B. undefine 'pandoc_path' and use 'pandoc_bin' in the default settings.

I updated the readme to instruct how to do the method B.