docs: templates in messages, closes #4

README.md

@@ -165,6 +165,36 @@ Processed errors:
 ```
 
 
+## Templates
+
+Custom error messages used in `errorMessage` keyword can be templates using [JSON-pointers](https://tools.ietf.org/html/rfc6901) or [relative JSON-pointers](http://tools.ietf.org/html/draft-luff-relative-json-pointer-00) to data being validated, in which case the value will be interpolated. Also see [examples](https://gist.github.com/geraintluff/5911303) of relative JSON-pointers.
+
+The syntax to interpolate a value is `${<pointer>}`.
+
+The values used in messages will be JSON-stringified:
+- to differentiate between `false` and `"false"`, etc.
+- to support structured values.
+
+Example:
+
+```json
+{
+  "type": "object",
+  "properties": {
+    "size": {
+      "type": "number",
+      "minimum": 4
+    }
+  },
+  "errorMessage": {
+    "properties": {
+      "size": "size should be a number bigger or equal to 4, current value is ${/size}"
+    }
+  }
+}
+```
+
+
 ## License
 
 [MIT](https://github.com/epoberezkin/ajv-errors/blob/master/LICENSE)

SPEC.md

@@ -78,7 +78,7 @@ console.log(ajv.errors);
 Limitation - it is not possible to define error messages for $ref keyword.
 
 
-## Templates // TODO
+## Templates // DONE
 
 Error messages used in errorMessage keyword can be templates using relative or absolute JSON pointers to data being validated, in which case the value will be interpolated. Primitive values (string, number, boolean, null) will be interpolated as is, structured values (array, object) will interpolated stable-stringified. Possibly, there can be a hook defined in options passed to `require('ajv-errors')` call to process interpolated values.