README for ignore_file_extension doesn't match behavior

[Yekut]

The ignore_file_extension behavior is changed in 025abf1c10f5d5991300c8810c580f115d6809e0 (and thanks for fixing @benwebber! took me a while to realize why 0.12.13 works differently from master), but the README.md doc wasn't updated to reflect that. OTOH, I can also imagine changing the implementation of ignore_file_extension to something more configurable (e.g., pass in a list to specify which extensions to ignore, or when True ignore the preset values as the current behavior), so filing a issue instead of a pull request for README.md.