Write "How to add a new attribute to semantic conventions"

[tigrannajaryan]

We would like to have "How to" guides to be part of our "Contributing" documentation. The guides need to be specific, describe the process from start to the end (success or rejection) for each particular class of changes (e.g. "How to add a semantic convention"). It is important for contributors to understand clear actions they can take to progress a contribution. The aim is to have minimum ambiguity, but also leave enough flexibility in the process to make it useful.