docs: improve the contributing section
docs: improve the contributing section

* Rephrased some paragraphs
* Better organised the section itself
* Added guideline and formatting examples for commit log messages

Signed-off-by: Hugo Hromic <ce89d3504d1ca35fd62a37545dd45feafe0b4199@gmail.com>

leon-anavi/meta-raspberrypi,agherzan/meta-raspberrypi,agherzan/meta-raspberrypi,schnitzeltony/meta-raspberrypi,agherzan/meta-raspberrypi,agherzan/meta-raspberrypi,agherzan/meta-raspberrypi,schnitzeltony/meta-raspberrypi,leon-anavi/meta-raspberrypi,leon-anavi/meta-raspberrypi,schnitzeltony/meta-raspberrypi,agherzan/meta-raspberrypi