Document contributing for newcomers

[dsalaza4]

Our contributing page can use some improvements for newcomers:

• How to run your local changes (git add for Makes to interpret new files)
• Adding yourself to the mailmap
• Valid commit message
• DCO
• Adding and rendering documentation (m . /docs/dev)

Anything else you might find important to include.

Let's try to make this as simple and direct as possible, action-oriented.

[dsalaza4]

@rohaquinlop You are the perfect individual for this! 😜

[rohaquinlop]

Can you please assign it to me?