docs: update src/README.md with src information and structure

src/README.md

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+# Python Package Source Code
+
+> [!NOTE]
+> This `src` folder houses the *source code* for the Python Package (including all modules, etc.)
+
+## Contents
+
+- [Overview](#overview)
+- [Structure](#structure)
+- [Notes](#notes)
+- [Reference](#reference)
+
+## Overview
+
+> [!NOTE]
+> This repository template uses the "src layout" for [Python Packaging](https://packaging.python.org/en/latest/discussions/src-layout-vs-flat-layout/).
+
+The "src layout" deviates from a "flat layout" by moving the source code that is intended to be importable (i.e. `import pkg`) into the `src` sub-directory.
+
+## Structure
+
+> [!NOTE]
+> The structure below is not exactly representative of this repository's structure but instead is a reference to a typical structure. 
+
+```plaintext
+src/
+├── README.md
+├── pkg/
+│   ├── __init__.py
+│   ├── __main__.py
+│   └── ...
+└── tools/
+    ├── __init__.py
+    ├── tool_one.py
+    ├── ...
+    └── tool_n.py
+```
+
+## Notes
+
+Notes about using a "src layout":
+
+- Requires installation of the project to be able to run its code.
+- Requires an additional step in the development workflow of the project (i.e. an [editable installation]() is used for development and a regular installation is used for testing).
+- Helps prevent accidental usage of the in-development copy of the codebase
+- Keeps import packages separate from root directory of the project, ensuring that the installed copy is used during testing and publication.
+
+## Reference
+
+- [src layout vs flat layout - Python Packaging User Guide](https://packaging.python.org/en/latest/discussions/src-layout-vs-flat-layout/)
+- [Packaging and distributing projects - Python Packaging User Guide](https://packaging.python.org/en/latest/guides/distributing-packages-using-setuptools/)