Add CloudFormation Create Stack button to README

[sfirrin]

What does this PR do?

• Adds a Create Stack button to installation instructions as the preferred option
• Updates installation language for clarity

Motivation

Make it easier to deploy

Testing Guidelines

Tested the button link works

Additional Notes

Types of changes

• [ ] Bug fix
• [ ] New feature
• [ ] Breaking change
• [x] Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• [x] This PR's description is comprehensive
• [ ] This PR contains breaking changes that are documented in the description
• [ ] This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• [ ] This PR impacts documentation, and it has been updated (or a ticket has been logged)
• [ ] This PR's changes are covered by the automated tests
• [ ] This PR collects user input/sensitive content into Datadog

[codecov-commenter]

Codecov Report

Merging #96 (edf6e9f) into master (04f5e9e) will not change coverage. The diff coverage is n/a.

:exclamation: Current head edf6e9f differs from pull request most recent head ed83536. Consider uploading reports for the commit ed83536 to get more accurate results

@@           Coverage Diff           @@
##           master      #96   +/-   ##
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  Coverage   86.43%   86.43%           
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  Files           8        8           
  Lines         612      612           
  Branches      153      153           
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  Hits          529      529           
  Misses         72       72           
  Partials       11       11           

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