Add README

[c24t]

I added a stub readme file in #16 to put contribution info front and center on the github project page, but the real readme file should include at least:

• a description of the OpenTelemetry project
• a description of the package structure
• installation and usage instructions
• links to examples and documentation

See also:

• https://github.com/open-telemetry/opentelemetry-java/blob/master/README.md
• https://github.com/census-instrumentation/opencensus-python/blob/master/README.rst
• https://github.com/opentracing/opentracing-python/blob/master/README.rst