[Documentation] create CONTRIBUTING.md

[lixuna]

Describe the update:

• create CONTRIBUTING.md for /certification
• ex. https://github.com/lfn-cnti/certification/blob/main/CONTRIBUTING.md

Describe why update is needed

• include info and links from https://wiki.lfnetworking.org/display/LN/Certification
• include link to https://wiki.lfnetworking.org/pages/viewpage.action?pageId=113213592
• include meeting times
• include slack channels

Related Examples

• https://github.com/lfn-cnti/bestpractices/blob/main/CONTRIBUTING.md
• https://github.com/cnti-testcatalog/testsuite/blob/main/CONTRIBUTING.md

Tasks

• [ ] Draft new CONTRIBUTING.md
• [ ] submit PR
• [ ] Peer review