Added more definition to the feedback section under the Collaboration value

Our culture is strong at being communicative and that's been working very well across and within teams.

I wanted to give some definition to giving feedback effectively and more specifically for managers providing feedback to their teams, as this was not covered at all in the handbook. A manager's feedback has critical impact on an employee's productivity, engagement, loyalty and happiness. For such an important aspect of the working relationship I felt this was a good start, backed by insights from various research stated in the two links I introduced, one by an MIT Sloan professor and another by IBM research.