Improvements on the structure of the first exercise
Improvements on the structure of the first exercise

I took the liberty to suggest a few improvements with this PR. Let me explain each a little.

1. I split it up in a "Requirement" and "Hint" section. Since this is the first section I called the "Hint" section "Introduction" because it focuses on Seneca in general rather than on the specifics of this exercise, the following exercises should call it "Hint". The Requirement contains all the necessary information to finish the exercise.
2. To make sure that people that `command` and `role` are `Seneca` idioms I highlighted them.
3. Consistent use of the word `microservice` and consistently referred to messages as JSON messages. 
4. Simplified attribution "just" instead of  "very simple, just"
5. Added an example for the Object specification
senecajs/seneca-in-practice,marcopiraccini/seneca-in-practice