Should I provide a LICENSE.txt or COPYING.txt file in my project? [closed]

COPYING is standard in the GNU project. I think that either of COPYING or LICENSE would be understood. The FSF has a guide on how to use the GPL, which suggests COPYING, while the Producing Open Source Software book suggests either COPYING or LICENSE. The Apache License recommends a boilerplate that just points to the license online, though the Apache project seems to also include a LICENSE file in their code containing the relevant license or licenses.

It's up to you whether you want to add the .txt suffix; on Windows and Mac OS X it tends to be helpful for default file associations, while on Linux or most other Unix-type operating systems suffixes are optional and it's generally assumed that files are plain text.