Add bug info for pandoc to README.md

Pandoc version 1.9.1.1, which is installed with `apt-get` on Ubuntu 12.04,
has a [bug](https://github.com/jgm/pandoc/issues/964) that hides a word
after tilde.  It is fixed with Pandoc version 1.11.1.
Merge pull request #561 from harupong/readme-pandoc

Add bug info for pandoc to README.md