Sklearn docs 2018 11 12

[ericangelokim]

Issue #, if available:

Description of changes:

Add Scikit-learn Estimator in Table of contents in root README.rst
Add Scikit-learn Estimator description next to other framework descriptions in README.rst
Fix Sciki-learn documenetation under /src/sagemaker/sklearn

Merge Checklist

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• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
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[codecov-io]

Codecov Report

Merging #545 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #545   +/-   ##
=======================================
  Coverage   92.77%   92.77%           
=======================================
  Files          71       71           
  Lines        5357     5357           
=======================================
  Hits         4970     4970           
  Misses        387      387

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