Add root README file

[lzchen]

The root README (used for OT registry), will contain all usage and documentation links.

The inner README will contain simple information and reference material used for Pypi documentation.

Follows similar design to: https://github.com/open-telemetry/opentelemetry-python

[codecov-io]

Codecov Report

Merging #77 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #77   +/-   ##
=======================================
  Coverage   98.57%   98.57%           
=======================================
  Files          14       14           
  Lines         774      774           
  Branches      102      102           
=======================================
  Hits          763      763           
  Misses          8        8           
  Partials        3        3           

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