Add issue assignment policy to contributor_docs

[adi-ray]

Fixes #3541

Changes: Added a clear bullet point to the "Before Submitting a Pull Request" section in contributor_docs/preparing_a_pull_request.md that explains the assignment policy.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123

[release-com]

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[raclim]

This looks great, thanks so much for your work on this!