Updated installation instructions

[SteliosZ]

I have updated the installation instructions to a more readable and understandable format.

I don't know if that's an appropriate change, but I kinda struggled with the existent instructions, so I decided to update them.

Thank you.

[0neday]

need sudo for mvn to install jnetpcap.jar
sudo mvn install:install-file -Dfile=jnetpcap.jar -DgroupId=org.jnetpcap -DartifactId=jnetpcap -Dversion=1.4.1 -Dpackaging=jar

and need install libpcap-dev, check here #133

sudo apt install libpcap-dev

and need jdk 1.8 for run ./gradlew

export JAVA_HOME=jdk1.8_path