docs: rename a file, to be less confusing

It sounded like an instigation to upgrade, or like a list of
instructions on how to upgrade.  But it is merely a list of
noteworthy changes -- what other packages put in the NEWS file.

Makefile.am

@@ -2,6 +2,6 @@ AUTOMAKE_OPTIONS = gnu no-dependencies
 
 SUBDIRS = doc m4 po src
 
-EXTRA_DIST = ChangeLog.pre-2.1 README.GIT UPGRADE nano.spec
+EXTRA_DIST = ChangeLog.pre-2.1 IMPROVEMENTS README.GIT nano.spec
 
 ACLOCAL_AMFLAGS = -I m4