Add build steps to contributing section

[Othmane Maniar]

add build sections under contributing

the section could look like this

What you will need

• Git
• Java 17 or higher
• Docker version 1.6.0 or higher

Get the Source Code

Clone the repository

git clone [e-mail]:spring-projects/spring-modulith.git
cd spring-modulith

Build the code

To compile, test, and build

./mvnw -B