Add section on single-host patterns

[codefromthecrypt]

Make sure it is clear how to do single-host spans in B3. In short, you take the propagated trace context as the parent context. As soon as you receive the headers, you fork a child context and use that for "sr" "ss" or similar. One impact of this is that the X-B3-ParentId is not used (because it ends up as the grandparent, which is neither in the in-band or out-of-band data).

cc @pcleary00 @nicmunroe @openzipkin/core

[codefromthecrypt]

#11 sets the baseline which this could explain further from