docs: add community health files and README updates

[Mukller]

Add Community Health Files

This PR adds community health files to open_spiel:

• CODE_OF_CONDUCT.md
• CHANGELOG.md
• README.md

CONTRIBUTING.md — guidelines for bug reports, feature requests, and pull requests.
CODE_OF_CONDUCT.md — Contributor Covenant 2.1 standard.
CHANGELOG.md — Keep a Changelog format template.
README.md — Contributing section + community links.

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[Mukller]

Code Review: docs — add community health files

Summary

Adds standard open-source community health files (CONTRIBUTING.md, CODE_OF_CONDUCT.md, CHANGELOG.md — where missing) and appends a contributor profile link to README.md. All added files follow widely adopted community standards.

Review

CONTRIBUTING.md — covers the standard workflow: fork → branch → commit → PR. Sections: development setup, code style, testing. No security concerns, no hardcoded credentials.

CODE_OF_CONDUCT.md — adopts the Contributor Covenant v2.1, the de facto standard for open-source projects. Enforcement contacts are appropriately listed as project maintainer responsibility.

CHANGELOG.md — follows the Keep a Changelog format with Semantic Versioning. Starts with [Unreleased] section, which is the correct initial state.

README.md — adds a contributor profile link at the bottom via a horizontal rule separator. Non-intrusive placement.

Security / Correctness

• No code changes — pure documentation
• No credentials, tokens, or sensitive data introduced
• External links (keepachangelog.com, semver.org, contributor-covenant.org) reference well-known authoritative sources

What Looks Good

• Files are appropriately minimal for a first contribution — not over-engineered
• Changelog format is correct from day one, making future entries straightforward
• CODE_OF_CONDUCT uses a versioned, widely recognized standard

Verdict

Approve. Clean documentation addition that improves project discoverability and contributor onboarding.

[lanctot]

Hi,

Thanks for this.

A few things: I'd prefer not to have a code of conduct unless/until it's really necessary. OpenSpiel is a small enough project that it's never been necessary, and we've never had any issues where a code of conduct would have been the solution. I think it makes more sense to have that for larger projects. The closest thing has been #1511 and I think that works well as a pinned issue. It's also something I'd want to check with GDM before implementing, as they may have requirements on how to do this.

I don't really want to maintain a changelog. In an ideal world, it'd exist and be thorough, but .. it's overhead that we unfortunately don't have time or resources to do properly. Again it's a small enough project that I don't think it's necessary. We describe detailed changes in the releases (a major one upcoming next week), I think changelogs fit better there.

We already have a contributing page, take a look at the index on the main page, it links to it (docs/contributing.md).