Install the NEWS and AUTHORS files

Fix #513

Makefile.am

@@ -7,7 +7,7 @@ pkgconfig_DATA = libmodbus.pc
 EXTRA_DIST = libmodbus.pc.in
 CLEANFILES += libmodbus.pc
 
-dist_doc_DATA = MIGRATION README.md
+dist_doc_DATA = MIGRATION README.md AUTHORS NEWS
 
 SUBDIRS = src doc