Add draft CHANGELOG

CHANGELOG.md

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 # Changelog
 
+## v1.2.0-rc.1 (December 2022)
+
+This is release v1.2.0-rc.1 of the OPTIMADE API specification.
+
+This minor release adds significant but optional new functionality to the specification, as well as providing several clarifications to existing behaviour.
+
+### New features
+
+- **Property definitions** ([#376](https://github.com/Materials-Consortia/OPTIMADE/pull/376)).
+A new section titled [Property Defintions](https://github.com/Materials-Consortia/OPTIMADE/blob/develop/optimade.rst#property-definitions) has been added to the specification which significantly extends the way in which implementations can define and describe the custom properties they serve, including URIs, unit definitions, API support levels (for querying and sorting) as well as full support for JSON Schema constructs for describing the JSON representation of the property.
+- **Files endpoint** ([#360](https://github.com/Materials-Consortia/OPTIMADE/pull/360)).
+The `/files` endpoint and corresponding [`files` entry
+type](https://github.com/Materials-Consortia/OPTIMADE/blob/develop/optimade.rst#files-entries) has been added to provide a robust way of linking entries to arbitrary file-based data relevant to the entry, such as alternative crystal structure representation formats, input or output files from computational procedures, or experimental data files.
+- **Boolean values** ([#348](https://github.com/Materials-Consortia/OPTIMADE/pull/348)).
+[Boolean values](https://github.com/Materials-Consortia/OPTIMADE/blob/develop/optimade.rst#comparisons-of-boolean-values) were overlooked in the first version of the filter grammar as no OPTIMADE fields required them.
+This functionality has been introduced for boolean fields using the syntax `TRUE` and `FALSE`.
+Only strict equality (`=`) and inequality (`!=`) comparisons on individual fields are supported.
+- **Fuzzy comparisons on lists** ([#415](https://github.com/Materials-Consortia/OPTIMADE/pull/415))
+String comparisons like `CONTAINS`, `STARTS WITH` and `ENDS WITH` are now compatible with list filter operations like `HAS`, `HAS ALL` etc.
+- **Backoff time** ([#411](https://github.com/Materials-Consortia/OPTIMADE/pull/411)):
+- **Database licenses** ([#414](https://github.com/Materials-Consortia/OPTIMADE/pull/414)):
+- **Symmetry data** ([#405](https://github.com/Materials-Consortia/OPTIMADE/pull/405)):
+
+
 ## v1.1.0 (July 8, 2021)
 
 This is release v1.1.0 of the OPTIMADE API specification.