Added requirements and usage information to the README.md.

Signed-off-by: RICCIARDI-Adrien <[email protected]>

README.md

@@ -3,3 +3,21 @@
 A Buildroot-based CI that allows checking radvd build against various CPU architectures and C libraries.  
 This repository contains the script to generate customized prebuilt Buildroot images that are used by a GitHub action in the radvd repository.  
 The prebuilt Buildroot images are automatically generated then stored in this repository [releases](https://github.com/radvd-project/radvd-ci/releases) section.
+
+## Requirements
+
+The script in this repository is designed to run on **Ubuntu 22.04**.  
+Make sure the following packages are installed :
+```
+sudo apt install build-essential curl git zstd
+```
+
+## Usage
+
+Run the `build-image.sh` script from your current user account (there is no need to be `root`) :
+```
+cd path/to/radvd-ci
+./build-image.sh <buildroot defconfig> <C library name> <output directory>
+```
+
+Run `build-image.sh` without parameter to display the embedded help.