Add CHANGELOG and document the update process

Add a CHANGELOG.md with an Unreleased section, set CHANGELOG.md to merge=union via .gitattributes, and document in CONTRIBUTING.md when and how to add entries.

Author: cconstable

.gitattributes

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+CHANGELOG.md merge=union

CHANGELOG.md

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+# Changelog
+
+## [Unreleased]
+
+<!--
+High-level release notes.
+Loosely based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
+
+When your PR includes a user-facing change, add an entry below under the
+appropriate heading (create the heading if it does not yet exist). Within
+each heading content can be free-form. Feel free to include examples, links
+to docs, or any other relevant information.
+
+### Added            — new features
+### Changed          — changes in existing functionality
+### Deprecated       — soon-to-be-removed features
+### Breaking Changes — removed or backwards-incompatible features
+### Fixed            — notable bug fixes
+### Security         — notable security fixes
+-->

CONTRIBUTING.md

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+# Contributing to the Temporal Python SDK
+
+Thanks for your interest in contributing!
+
+All contributors must complete the Temporal Contributor License Agreement (CLA) before changes
+can be merged. A link to the CLA will be posted in the PR.
+
+See the [README](README.md) for build and development instructions.
+
+## Changelog
+
+User-facing changes are recorded in [`CHANGELOG.md`](CHANGELOG.md), loosely following the
+[Keep a Changelog](https://keepachangelog.com/en/1.1.0/) format.
+
+If your PR includes a user-facing change (new feature, behavior change, deprecation, breaking
+change, notable bug fix, or security fix), add a short, high-level entry to the `## [Unreleased]`
+section at the top of `CHANGELOG.md` under the appropriate heading, creating it if needed:
+Added, Changed, Deprecated, Breaking Changes, Fixed, or Security.
+
+Keep entries high-level and written for users. The full commit log is appended at release time,
+so internal-only changes (refactors, tests, CI, docs) don't need an entry.