(Info found on superuser.com)
In Windows 7, to associate a default program to files without extension (eg. README or INSTALL) you need to
cmd.exe
as administrator>assoc .="No Extension"
>ftype "No Extension"="C:\Program Files (x86)\TextPad 4\TextPad.exe" "%1"
>assoc "No Extension"\DefaultIcon=%SystemRoot%\System32\imageres.dll,-102
Obviously, adapt the path to the editor you want to use.