add some minimal usage examples

swig/run_perl_example.sh

@@ -0,0 +1,17 @@
+#!/bin/sh
+
+cat > example.pl <<EOF
+use pllammps;
+
+\$lmp = pllammps::lammps_open_no_mpi(0,undef,undef);
+\$ver = pllammps::lammps_version(\$lmp);
+
+print("LAMMPS version ",\$ver,"\n");
+pllammps::lammps_close(\$lmp)
+EOF
+
+PERL5LIB=$PWD:${PERL5LIB-PWD}
+
+export PERL5LIB
+
+perl example.pl