Add Zarr Geo Projection Attribute Extension specifications and schema (#39)

* Add Zarr Geo Projection Attribute Extension specifications and schema

* Update attributes/README.md

Co-authored-by: Davis Bennett <[email protected]>

* Update attributes/README.md

Co-authored-by: Davis Bennett <[email protected]>

* Update attributes/geo/proj/README.md

Co-authored-by: Davis Bennett <[email protected]>

* Add implementation requirement to attribute specifications and enhance geo/proj documentation with versioning details

* Remove status and registry entry from Geo Projection Attribute Extension section in README.md

---------

Co-authored-by: Davis Bennett <[email protected]>

Author: emmanuelmathot

attributes/README.md

@@ -0,0 +1,36 @@
+# Zarr Attributes Specifications
+
+This directory contains specifications for encoding Geospatial metadata in Zarr hierarchies.
+
+## Overview
+
+Zarr attributes are metadata stored in the `attributes` object in Zarr array and group metadata documents. While the `attributes` can contain arbitrary metadata, this directory provides specifications for commonly used attribute conventions within the Zarr community. These specifications also define the structure of a Zarr hierarchy, e.g. the layout of groups, the names of the nodes in the hierarchy, and the properties of the arrays.
+
+## Registered Attributes
+
+### [`geo/proj`](./geo/proj/README.md) - Geo Projection Attribute Extension
+
+Defines a JSON object that encodes datum and coordinate reference system (CRS) information for geospatial data stored under the `proj` key within the `geo` dictionary.
+
+**Key Features**:
+
+- Simple, standardized CRS encoding without complex nested structures
+- Compatible with existing geospatial tools (GDAL, rasterio, pyproj)
+- Based on the proven STAC Projection Extension model
+- Group-to-array inheritance model for CRS metadata
+- Support for multiple CRS representations (EPSG codes, WKT2, PROJJSON)
+
+**Specification**: [./geo/proj/README.md](./geo/proj/README.md)
+
+## Contributing
+
+For changes to attribute specifications:
+
+1. **Content Changes**: Submit PRs to this repository for specification content
+2. **Registration Changes**: Submit PRs to [zarr-extensions](https://github.com/zarr-developers/zarr-extensions) for registry updates
+3. **New Attributes**: Follow the [registration process](https://github.com/zarr-developers/zarr-extensions#registering-an-attribute) in zarr-extensions
+4. **Implementation Requirement**: Every specification should include a reference implementation in this repository to ensure the spec can be practically implemented
+
+## License
+
+All attribute specifications are licensed under the [Creative Commons Attribution 3.0 Unported License](https://creativecommons.org/licenses/by/3.0/), consistent with the zarr-extensions registry.