clarify structure of the code of conduct
clarify structure of the code of conduct

after proposing the adoption of the CoC on the #scala IRC channel,
I got some feedback from a few people that the "Be excellent"
part was... a little weird.

I've addressed that as follows in two ways:

1) Replacing the "be excellent" wording (because it sounds too much
   like Bill and Ted)

2) Separating the four points of the code into "rules" and
   "guidelines", so it's clearer which parts of the code are about
   what is actually allowed

andy1138/scala-lang,andy1138/scala-lang,jeantil/scala-lang,jeantil/scala-lang,jeantil/scala-lang,andy1138/scala-lang,jeantil/scala-lang,andy1138/scala-lang