guidelines: create documentation guidelines

[diegodelemos]

First, we should identify our audience(s) then write a set of easy to follow rules to remove any doubt from the mind documentation writer's mind and focus on the message to be conveyed to that(these) audience(s). For example:

• https://docs.djangoproject.com/en/2.2/internals/contributing/writing-documentation/
• https://kubernetes.io/docs/contribute/style/style-guide/
• https://www.divio.com/blog/documentation/
• See also previous attempts in Invenio docs: how to write documentation inveniosoftware/invenio#3948

We should keep it as simple as possible