New section about designing a feature in the workflow

The change includes a new section "Designing a feature" with
the details on how to do a design, and a template for it.

It also clarifies some of the aspects related to the proposal and
approval of a feature, and its later implementation.

Finally it also includes minor changes to improve readability, like
proper Markdown indentation in different parts of the workflow.

Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>