update the meta-schema to allow for extension; add readme with basic how-to

Author: gregsdennis

specs/meta/README.md

@@ -0,0 +1,74 @@
+# JSON Schema Meta-Schema
+
+The _meta.json_ file contains the meta-schema for the in-progress JSON Schema
+specifications. You can find the latest published version on the
+[JSON Schema website](https://json-schema.org).
+
+## Meta-Schemas
+
+A meta-schema is a schema that itself describes a schema. This is possible
+because JSON Schema can be written as JSON.
+
+Furthermore, the JSON Schema meta-schema is self-validating. That is, its JSON
+form validates against the meta-schema.
+
+## Extensions and Unknown Keywords
+
+The JSON Schema specification allows for extension keywords to be introduced,
+however it also disallows unknown keywords. While seemingly contradictory, the
+meta-schema has been set up to allow for extension.
+
+For this example, we'll add two hypothetical keywords: `odds` and `evens`, which
+validate the odd and even indexed values in an array, respectively.
+
+First, create a schema that just defines the new keywords.
+
+```json
+{
+  "$schema": "https://json-schema.org/1",
+  "$id": "https://example.com/schema/odds-evens",
+
+  "properties": {
+    "odds": { "$dynamicRef": "#meta" },
+    "evens": { "$dynamicRef": "#meta" }
+  }
+}
+```
+
+Second, create a new meta-schema that
+
+- references the above schema
+- references the JSON Schema meta-schema
+- includes an extension point in a `$defs`
+
+```json
+{
+  "$schema": "https://json-schema.org/1",
+  "$id": "https://example.com/schema/odds-evens-extension",
+
+  "$ref": "https://json-schema.org/1",
+
+  "$defs": {
+    "extension": {
+      "$dynamicAnchor": "extension",
+      "$ref": "https://example.com/schema/odds-evens"
+    }
+  }
+}
+```
+
+Now you can use `https://example.com/schema/odds-evens-extension` in your
+schemas to make use of the new `odds` and `evens` keywords.
+
+```json
+{
+  "$schema": "https://example.com/schema/odds-evens-extension",
+  "$id": "https://example.com/schema/model",
+
+  "type": "array",
+  "odds": { "type": "string" },
+  "evens": { "type": "number" }
+}
+```
+
+This schema will validate arrays whose items alternate between strings and numbers.

specs/meta/meta.json

@@ -152,7 +152,12 @@
     "propertyNames": {
       "pattern": "^[^$]|^\\$(id|schema|ref|anchor|dynamicRef|dynamicAnchor|comment|defs)$"
     },
+    "$dynamicRef": "#extension",
+    "unevaluatedProperties": false,
     "$defs": {
+        "extension": {
+            "$dynamicAnchor": "extension"
+        },
         "anchorString": {
             "type": "string",
             "pattern": "^[A-Za-z_][-A-Za-z0-9._]*$"