Add PR size guidelines to CONTRIBUTING.md

[pmcollins]

Description

This PR adds guidance on keeping pull requests small and focused to improve the review process.

Recommends 500 lines of additions or fewer, which comes from the Collector's CONTRIBUTING.md.

Fixes # (no issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

N/A - Documentation-only change. Verified markdown renders correctly.

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR: (to be created after this PR is merged)
• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated (this PR IS a documentation update)

[shuwpan]

Totally agree

[JWinermaSplunk]

nitpick: I might change If you do this and want the community to understand how your small PR fits into the context of your larger change, to If you want the community to understand how your small PR fits into the context of your larger change, and remove the second incremental from Regardless, strive to provide clear, incremental value to the repository for every incremental change you make, but that is just personal preference. Otherwise, LGTM!

[JWinermaSplunk]

PR probably also does not need a CHANGELOG

[pmcollins]

Thanks @emdneto -- I made a few additional tweaks since your suggestion and approval.