Acknowledge Lucas Gruss for commit 1bbc076

This adds initial support for 'tab-line-mode' faces. It was done in
pull request 6: <#6>

The change is within the ~15 line limit. Lucas does not need to assign
copyright to the Free Software Foundation.

README.org

@@ -267,7 +267,7 @@ help matters.
 
 + Author/maintainer :: Protesilaos Stavrou.
 
-+ Contributions to code or the manual :: Neo Dim.
++ Contributions to code or the manual :: Lucas Gruss, Neo Dim.
 
 + Ideas and/or user feedback :: Damien Cassou, Nicolas Semrau, Tomasz
   Hołubowicz.