Doc: add a clearly visible note to create a cfg via FromEnv() if env vars should be active to the readme

[Lercher]

Requirement - what kind of business use case are you trying to solve?

Using configuration environment variables as documented is only active if the configuration object is initialized in a special way.

Problem - what in Jaeger blocks you from solving the requirement?

The section explaining the vars has no clearly visible note that it depends on initializing the cfg object by FromEnv and this can be overlooked easily. Moreover the examples in the code file mentioned start with a method that ignores env vars.

Proposal - what do you suggest to solve the problem or improve the existing situation?

Add to the first sentence in https://github.com/jaegertracing/jaeger-client-go#environment-variables something like: ... if the tracer is created from a configuration object that was created via FromEnv() or via ...

Any open questions to address

The godoc for all configuration fields states the env var to be active without a condition, i.e. without referencing to FromEnv. Is it helpful to add such hints?

[yurishkuro]

feel free to propose a PR