Add hello world documentation

docs/SUMMARY.md

@@ -2,3 +2,4 @@
 
 - [Introduction](./introduction/SUMMARY.md)
   - [Installation](./introduction/installation.md)
+  - [Hello World!](./introduction/hello-world.md)

docs/introduction/hello-world.md

@@ -0,0 +1,45 @@
+# The command line interface
+
+Now that you have installed Sabre, it is time to write our first program. This is a program that will simply print a string to the screen.
+
+> **Note:** Sabre is still at a very early state of development. Currently, the only way to print something to stdout is by returning a value from the `main` function.
+
+# Creating a project directory
+
+Let's begin by setting up our development workspace. Sabre really doesn't care where you store the code, so feel free to choose a different directory, than the one in this example.
+
+```
+mkdir ~/sources
+cd ~/sources
+mkdir hello_world
+cd hello_world
+```
+
+# Writing and running a program
+
+Next, make a new source file and call it `main.sb`. Sabre files should always end with `.sb` by convention.
+
+Now open the main.sb file you just created and enter the following code:
+
+```
+fn main() {
+    return "Hello, world!"
+}
+```
+
+Save the file and go back to your terminal window. Now, run the following command to compile your program:
+
+```
+sabre build main.sb -o main.js
+```
+
+> **Note:** JavaScript is the only currently supported backend for Sabre. There will be more soon.
+
+Now, execute the compiled program with a runtime of your choice. We will be using Node.js for this example.
+
+```
+> node main.js
+Hello, world!
+```
+
+You should see the string `Hello World!` on the screen. Congrats! You have officially written a Sabre Program!