Add changelog

Author: benwebber

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+### Added
+
+- Added `index_phrases` (`bool`) and `index_prefixes` (`IndexPrefixes`) fields to `FieldMapping`.
+- Added `boost` (`double`) field to `FieldMapping`.
+- Added `Target` message (renamed from `OutputTarget`) with `label` and `json` fields for specifying literal per-target mappings.
+- Added error and warning diagnostics.
+
+  Any error aborts compilation.
+- Validate field names match conventional naming rules (`W001`).
+- Validate `ignore_above`.
+
+  Emit `E001` when the value is less than or equal to zero.
+- Validate `position_increment_gap`.
+
+  Emit `E001` for negative values.
+- Validate `index_prefixes.min_chars` and `index_prefixes.max_chars`.
+
+  Emit `E001` for negative `min_chars` or `max_chars` outside `0..=20`.
+- Emit `W002` when a `target=` label does not match a known target.
+- Emit `E002` or `E003` when a `target` entry's `json` is not valid JSON or is not a JSON object.
+- Validate plugin parameters (`target=<label>`).
+
+  Previously, unknown parameters were silently ignored. Now, they cause a fatal error.
+
+### Changed
+
+- **BREAKING:** Renamed the top-level extension field from `(protosearch.field)` to `(protosearch.mapping)`. Moved field parameters to the `(protosearch.mapping).field` field.
+- **BREAKING:** Replaced the `dynamic`, `index_options`, and `term_vector` string fields with enum types.
+- **BREAKING:** Moved output `name` and `target` to `Mapping`.
+- Print warnings to standard error. Only return fatal errors to `protoc` using `set_error()`.
+
+### Fixed
+
+- Corrected `fielddata` type from `google.protobuf.Value` to `bool`.
+- Corrected `subobjects` type from `string` to `bool`.
+
+[Unreleased]: https://github.com/benwebber/protosearch/compare/v0.1.0...HEAD