docs: add CONTRIBUTING.md with coding conventions and review guide

[erhankur]

The guide is based on patterns already followed across the codebase. It covers naming, weak/strong overrides,
error handling, ROM functions, memory rules, logging, SOC headers, CMake integration, testing, and code style.

This is the first version. It covers the essentials but is open to discussion and suggestions.

[Copilot]

Pull request overview

Adds a first-version contributing guide and links it from the main README to standardize coding conventions and review expectations for esp-stub-lib.

Changes:

• Add CONTRIBUTING.md documenting naming, weak/strong overrides, error handling, ROM usage, memory/logging rules, SOC headers, CMake patterns, and review checklist.
• Link the new contributing guide from the README “Contributing” section.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
README.md	Adds a pointer to the new contributing guide.
CONTRIBUTING.md	Introduces a comprehensive contribution/conventions document and reviewer checklist.

[Copilot]

Fix missing space after the period: esptool.They should be esptool. They.

Suggested change

	- Avoid large `static` variables. The library serves both OpenOCD and esptool.They
	- Avoid large `static` variables. The library serves both OpenOCD and esptool. They

[Copilot]

This section is describing static variables (statics), not statistics. Renaming the heading and the last sentence (e.g., “### Statics” and “prefer parameters over statics”) would avoid confusion for contributors.

[Copilot]

Hyphenate the compound modifier: Compile time → Compile-time.

Suggested change

	Compile time conditional (`STUB_LOG_ENABLED`). Use the macros:
	Compile-time conditional (`STUB_LOG_ENABLED`). Use the macros:

[github-actions]

	Messages
📖	🎉 Good Job! All checks are passing!

👋 Hello erhankur, we appreciate your contribution to this project!

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[dobairoland]

@erhankur Thank you very much for taking the time and making this. Awesome. I don't have any specific requirement for the rules - I'm happy that they are defined and will help all future contributors.

Please consider linking the document from the copilot config file so it would use it for reviews.