Fixed Time Grain and Added Drill to README

[cgivre]

CATEGORY

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• Bug Fix
• Enhancement (new features, refinement)
• Refactor
• Add tests
• Build / Development Environment
• Documentation

SUMMARY

This PR fixes a small bug in the time grains for Apache Drill and adds Drill to the README file.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TEST PLAN

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Requires DB Migration.
• Confirm DB Migration upgrade and downgrade tested.
• Introduces new feature or API
• Removes existing feature or API

REVIEWERS

[villebro]

Total nit, but while we're at it, perhaps nearestdate should be NEARESTDATE in all grains as per typical SQL uppercasing convention?

[codecov-io]

Codecov Report

Merging #7635 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #7635   +/-   ##
=======================================
  Coverage   65.53%   65.53%           
=======================================
  Files         435      435           
  Lines       21726    21726           
  Branches     2393     2393           
=======================================
  Hits        14239    14239           
  Misses       7365     7365           
  Partials      122      122

Impacted Files	Coverage Δ
superset/db_engine_specs.py	67.23% <ø> (ø)	⬆️

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[cgivre]

@villebro
Fixed.