Add temporary installation instructions

[nfahlgren]

Describe your changes
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What was the previous functionality (if relevant) and what can we do now with
these changes.

Type of update
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• Bug fix
• New feature or feature enhancement
• Update to documentation
• Work in progress

Associated issues
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Additional context
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For the reviewer
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• PR functionality reviewed in a Jupyter Notebook
• All tests pass
• Test coverage remains 100%
• Documentation tested
• New documentation pages added to plantcv/mkdocs.yml
• Changes to function input/output signatures added to updating.md
• Code reviewed
• PR approved

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