add prefix chunk key encoding

chunk-key-encodings/prefix/README.md

@@ -0,0 +1,51 @@
+# `prefix` Chunk Key Encoding
+
+## Summary
+
+This specification defines a chunk key encoding that prepends a constant string to the chunk keys produced by another chunk key encoding.
+
+## Metadata
+
+| field | type | required |
+| - | - | - |
+| `name` | Literal "prefix" | yes |
+| `configuration` | [Configuration](#configuration) | yes |
+
+## Configuration
+
+| field | type | required | 
+| - | - | - |
+| [`prefix`](#prefix) | string | yes |
+| [`base_encoding`](#base-encoding) | chunk key encoding | yes |
+
+### `prefix`
+
+The `prefix` field is a string that is prepended to a chunk key generated by the chunk key encoding declared in the `base_encoding` field.
+
+The following constraints apply to the `prefix` field:
+- `prefix` MUST NOT begin with `/`.
+- `prefix` MUST NOT contain the substrings `..`, `\`, `\0`, or `//`.
+- `prefix` MUST NOT contain the string `zarr.json`.
+- `prefix` MUST NOT be the empty string.
+
+### `base_encoding`
+
+The `base_encoding` field is the metadata for another chunk key encoding. The output of this chunk key encoding is the base string to which the string declared by `prefix` is prepended.
+
+## Examples
+
+### x/
+
+This configuration prepends `x/` to the key generated by the `v2` chunk key encoding with the default configuration (`separator` implicitly set to `.`).
+
+```json
+{
+    "name": "prefix",
+    "configuration": {
+        "prefix": "x/",
+        "base_encoding": "v2"
+    }
+}
+```
+
+For a 3-dimensional array, the chunk at (0,0,0) would have the key `x/0.0.0`.