How to unignore files in certain directories?

Hi,

I have the following in .gitignore to ignore *.txt files.

*.txt

But I want to keep the *.txt files in, for example, data/ and all its
subdirectories. I don't know should the the correct way to unignore
these *.txt files. Could anybody show me the command that I should add
to .gitignore? Thanks!

-- 
Regards,
Peng