I'm trying to use `pandoc` (version 1.19.2.4) on Ubuntu 18.04 to render github flavoured markdown with:

`pandoc --from gfm --to html README.md`

This gives the error `pandoc: Unknown reader: gfm`.

I guess I need to verify/install `gfm`, but its not clear how to do this.

How do I fix this error? How do I use pandoc to generate github flavoured markdown?