README.md: Note autoreconf need only in unprepared tarball

Author: scop

README.md

@@ -23,7 +23,7 @@ you simply don't want to use one, you can install bash completion using the
 standard commands for GNU autotools packages:
 
 ```shell
-autoreconf -i
+autoreconf -i  # if not installing from prepared release tarball
 ./configure
 make
 make check # optional, requires dejagnu and tcllib