[WIP] Readme updates

[sueann]

WIP PR that needs a bit more work.

Work so far:

• some minor wordsmithing edits
• a more comprehensive SQL UDF example
• added links to release-specific Databricks notebooks so users know what works in each release.

Needs:

• a new link to the Databricks notebook for Release 0.3.0 once that notebook has been updated
• reorganize the user guide section in README.md here match the new organization in the 0.3.0 notebook (mostly shuffling the TensorFlow and Keras Transformer sections around) which also adds a TFTransformer example

[codecov-io]

Codecov Report

Merging #104 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #104   +/-   ##
=======================================
  Coverage   82.84%   82.84%           
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  Files          34       34           
  Lines        1999     1999           
  Branches       44       44           
=======================================
  Hits         1656     1656           
  Misses        343      343

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[sueann]

Done in #106