update looper docs

Author: nsheff

docs/looper/changelog.md

@@ -2,6 +2,13 @@
 
 This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html) and [Keep a Changelog](https://keepachangelog.com/en/1.0.0/) format.
 
+## [Unreleased]
+
+### Added
+- Added `inject_env_vars` pipeline interface property for injecting environment variables into submission scripts
+- Added `pipestat_config_required` pipeline interface property to control pipestat handoff validation
+- Added validation that pipestat-enabled interfaces (with `output_schema`) pass config to the pipeline via CLI (`{pipestat.*}`) or environment variable (`PIPESTAT_CONFIG` in `inject_env_vars`)
+
 ## [2.0.3] -- 2025-09-23
 ### Fixed
 - Fixed [#543](https://github.com/pepkit/looper/issues/543)