fix: add readme for plugin

Author: kulikp1

README.md

@@ -0,0 +1,57 @@
+# JSON Editor
+
+The JSON Editor plugin replaces the default text input for JSON columns with a structured key-value editor. Users can add, edit, and delete individual pairs without writing raw JSON.
+
+Values support all standard JSON types: strings, numbers, booleans, `null`, arrays, and nested objects.
+
+## Installation
+
+```bash
+pnpm install @adminforth/json-editor --save
+```
+
+## Setting up
+
+Import and attach the plugin to an existing column in your resource:
+
+```ts title="./your_resourse"
+//diff-add
+import JsonEditorPlugin from '@adminforth/json-editor';
+
+export default {
+  ...
+  plugins: [
+    ...
+//diff-add
+    new JsonEditorPlugin({
+//diff-add
+      fieldName: 'description',
+//diff-add
+    }),
+  ],
+}
+```
+
+> The plugin works on columns with type `json`, `string`, or `text`. The stored value must be a JSON object (`{}`). Top-level arrays and primitives are not supported.
+
+## Value types
+
+Values are entered in JSON notation:
+
+| Input | Saved as |
+|---|---|
+| `"hello"` | string |
+| `42` | number |
+| `true` / `false` | boolean |
+| `null` | null |
+| `[1, "x", true]` | array |
+| `{"key": 1}` | nested object |
+
+## Validation
+
+Saving is blocked when:
+
+- **Duplicate keys** - two rows share the same key name.
+- **Invalid JSON value** - the value field is not valid JSON (e.g. `hello` without quotes). The error identifies the row: `Row 2: value is not valid JSON`.
+
+An empty value field is allowed and saved as an empty string `""`.