Move MIGRATION and README.md to dist_doc_DATA target

Makefile.am

@@ -1,11 +1,12 @@
-EXTRA_DIST = MIGRATION README.md
 CLEANFILES =
 ACLOCAL_AMFLAGS = -I m4 ${ACLOCAL_FLAGS}
 AM_MAKEFLAGS = --no-print-directory
 
 pkgconfigdir = $(libdir)/pkgconfig
 pkgconfig_DATA = libmodbus.pc
-EXTRA_DIST += libmodbus.pc.in
+EXTRA_DIST = libmodbus.pc.in
 CLEANFILES += libmodbus.pc
 
+dist_doc_DATA = MIGRATION README.md
+
 SUBDIRS = src tests doc