Exclude from source archives files specific to Git work tree

Add the "export-ignore" attribute to all such files.  Remove the
suggestion from the README since this should not be a responsibility
of the including project.

.gitattributes

@@ -1,4 +1,7 @@
 .git*            export-ignore
 
+# Exclude from source archives files specific to Git work tree.
+*                export-ignore
+
 tips             eol=lf         whitespace=indent-with-non-tab
 setup-*          eol=lf         whitespace=indent-with-non-tab

README

@@ -37,13 +37,6 @@ Commit the merge with an informative message:
  the general GitSetup repository "setup" branch.
  ------------------------------------------------------------------------
 
-Optionally add to the project ".gitattributes" file the line
-
- /Utilities/GitSetup export-ignore
-
-to exclude the GitSetup directory from inclusion by "git archive"
-since it does not make sense in source tarballs.
-
 
 Configuration
 -------------