Add README section for hyperparameter tuning jobs

[laurenyu]

Description of changes:
First go at some documentation for hyperparameter tuning job support in our README. I plan on adding some links to example notebooks, and I know some of the links are currently broken.

Merge Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your pull request.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have updated the changelog with a description of my changes (if appropriate)
• I have updated any necessary documentation (if appropriate)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-io]

Codecov Report

Merging #216 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #216   +/-   ##
=======================================
  Coverage   91.49%   91.49%           
=======================================
  Files          45       45           
  Lines        3163     3163           
=======================================
  Hits         2894     2894           
  Misses        269      269

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 524a8ce...d9c9e12. Read the comment docs.