Add instructions for adding plugins in .zshrc.d files

[unixorn]

License Acceptance

• This repository is Apache version 2.0 licensed (some scripts may have alternate licensing inline in their code) and by making this PR, I am contributing my changes to the repository under the terms of the Apache 2 license.

Description

Per #205, update instructions on using your own plugins, either in addition to the kit's standard load, or to replace the standard list.

Type of changes

• A helper script
• A link to an external resource like a blog post or video
• Text cleanups/updates
• Test updates
• Bug fix
• New feature
• Plugin list change

Checklist

• I have read the CONTRIBUTING document.
• I have updated the readme if this PR changes/updates quickstart functionality.
• All new and existing tests pass.
• Any scripts added use #!/usr/bin/env interpreter instead of potentially platform-specific direct paths (#!/bin/sh is an allowed exception)
• Scripts are marked executable
• Scripts do not have a language file extension unless they are meant to be sourced and not run standalone. No one should have to know if a script was written in bash, python, ruby or whatever. Not including file extensions makes it easier to rewrite the script in another language later without having to change every reference to the previous version.
• I have added a credit line to README.md for the script
• If there was no author credit in a script added in this PR, I have added one.
• I have confirmed that the link(s) in my PR are valid.

[github-actions]

MegaLinter status: ⚠️ WARNING

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ COPYPASTE	jscpd	yes		no	1.44s
⚠️ MARKDOWN	markdownlint	1		1	0.72s
✅ MARKDOWN	markdown-table-formatter	1		0	0.4s
✅ REPOSITORY	checkov	yes		no	5.77s
✅ REPOSITORY	devskim	yes		no	0.9s
✅ REPOSITORY	dustilock	yes		no	0.01s
✅ REPOSITORY	gitleaks	yes		no	0.42s
✅ REPOSITORY	git_diff	yes		no	0.01s
✅ REPOSITORY	secretlint	yes		no	1.05s
✅ REPOSITORY	syft	yes		no	0.47s
✅ REPOSITORY	trivy	yes		no	3.32s
✅ SPELL	misspell	1		0	0.11s

See errors details in artifact MegaLinter reports on CI Job page
Set VALIDATE_ALL_CODEBASE: true in mega-linter.yml to validate all sources, not only the diff