Be more explicit about skills needed to write a good tracer

[codefromthecrypt]

In http://zipkin.io/pages/instrumenting.html, our first section says "This is an advanced topic" However, it might not be clear what parts are advanced and what aren't. Particularly if "We’ll be extremely happy to help you along the way." :)

For example, B3 (adding headers) is relatively simple and you don't need much background to succeed doing that. You can also know the basic model and architecture without much experience.

However, designing a complete tracer library is definitely advanced regardless of how we structure the docs. For example, if you've not written anything similar (such as an asynchronous logger or metrics reporter), or don't have a networking background, a lot of this will be hard to do. Moreover, libraries started without multiple contributors are often abandoned. Without the right experience and community, it may not be a great idea to try to create a tracer.

My take is that we should change our http://zipkin.io/pages/instrumenting.html page to be more realistic.

• Make our instrumentations page more clear about what's actually supported vs just out there.
• Discourage or at least don't imply blank-check support for rewriting supported libraries.
• Reword the instrumenting page to be more clear about prerequisites helpful when making a copy of projects like zipkin-js