Update readme

Author: dusterherz

README.md

@@ -3,3 +3,18 @@
 An hello world for Game Boy made with GBZ80 ASM.
 
 This has been done with the help of [@ISSOtm](https://github.com/ISSOtm) tutorial on how to dev on Gameboy in ASM Z80. (You can [find it here](https://eldred.fr/gb-asm-tutorial/index.html))
+
+## Get the rom
+
+The rom can be find in the release of this project.
+
+## Build the project
+
+You can build the project with the help of `RGBASM`. You can install it [following this instructions](https://github.com/rednex/rgbds#1-installing-rgbds).
+Then, you can build the project doing :
+
+```sh
+make all
+```
+
+And tada ! A `hello-world.gb` rom should have appear, ready to be used on an emulator or on a flash cartrige.