Document default values

[pavolloffay]

Related to #11 we should also document the default values of properties that have default values.

[jcchavezs]

Default values are per language so I am not sure if this issue is related to this config or to the specs folder in the main repo?

[pavolloffay]

There are a couple of fields that we might consider using the same defaults across languages:

• reporting address shall we default to the localhost?
• service name unknown or unknown_language?
• propagation protocol