Updates postproc README to include Kestrel environment setup

Pull request overview

Updates postprocessing README to include Kestrel HPC environment creation, which uses python's venv instead of conda.

This pull request makes changes to (select all the apply):

[ ] Documentation
[ ] Infrastructure (includes singularity image, buildstock batch, dependencies, continuous integration tests)
[ ] Sampling
[ ] Workflow Measures
[ ] Upgrade Measures
[ ] Reporting Measures
[X] Postprocessing

Author pull request checklist:

[ ] Tagged the pull request with the appropriate label (documentation, infrastructure, sampling, workflow measure, upgrade measure, reporting measure, postprocessing) to help categorize changes in the release notes.
[ ] Added tests for new measures
[ ] Updated measure .xml(s)
[ ] Register values added to comstock_column_definitions.csv
[ ] Both options_lookup.tsv files updated
[ ] 10k+ test run
[ ] Change documentation written
[ ] Measure documentation written
[ ] ComStock documentation updated
[ ] Changes reflected in example .yml files
[X] Changes reflected in README.md files
[ ] Added 'See ComStock License' language to first two lines of each code file
[ ] Implements corresponding measure tests and indexing path in test/measure_tests.txt or/and test/resource_measure_tests.txt
[ ] All new and existing tests pass the CI

This will not be exhaustively relevant to every PR.

[ ] Perform a code review on GitHub
[ ] All related changes have been implemented: data and method additions, changes, tests
[ ] If fixing a defect, verify by running develop branch and reproducing defect, then running PR and reproducing fix
[ ] Reviewed change documentation
[ ] Ensured code files contain License reference
[ ] Results differences are reasonable
[ ] Make sure the newly added measures has been added with tests and indexed properly
[ ] CI status: all tests pass

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# See top level LICENSE.txt file for license terms.