GH-18: Add CONTRIBUTING.md guidelines

[rzr]

This text has been adapted from silabs internal development to github

Bug-SiliconLabs: UIC-3082

Change

Some guidelines as suggested

Checklist

• [x] A Contribution License Agreement has been established between @SiliconLabs and author's company (matching email domain)

[rzr]

Some hints here for our new contributors: @nkljajic

Suggestions are always welcome