fix #1: adds the hello world program

Makefile

@@ -0,0 +1,4 @@
+all: run
+
+run:
+	./hello.sh

README.md

@@ -1,2 +1,21 @@
 # hello-world
-its a program that greets worlds
+
+This is a program that greets worlds.
+
+## building and running
+
+Just write:
+
+``` shell
+$ make
+```
+
+Or even:
+
+```sh
+$ ./hello.sh mars
+hello, mars
+$ ./hello.sh terra
+hello, terra
+```
+

hello.sh

@@ -0,0 +1,8 @@
+#!/usr/bin/env bash
+
+hello_world() {
+  local world=${1:-world}
+  echo "hello, $world"
+}
+
+hello_world $1