usage article in README.md added

README.md

 # JSON Schema form generator RenderJS gadget
 
+## Usage
+
+It's assumed that a developer is familiar with RenderJS gadget documentation (https://renderjs.nexedi.com/).
+
+Gadget api is similar to any renderjs field api.
+
+```javascript
+gadget.render({
+  key: 'field_identificator',
+  value: [1,2],
+  schema: {
+    type: 'array',
+    items: {
+      type: 'integer'
+    }
+  }
+})
+```
+
+![rendered](doc/array_rendered.png)
+
+If schema_url is specified instead of schema property then schema_url is used for schema download.  
+
+Schema_url parameter should be specified for proper computation of an absolute url. If schema_url is omitted
+then the current gadget url is used as base_url. 
+
+Parent gadget should have declared notification methods to catch validation status.
+
+```javascript
+rJS(window)
+  .allowPublicAcquisition("notifyValid", function (arr, scope) {
+  })
+  .allowPublicAcquisition("notifyInvalid", function (arr, scope) {
+  })
+  .allowPublicAcquisition("notifyChange", function (arr, scope) {
+  });
+```
+
+In parent gadget downloadJSON() can be declared. DownloadJSON() can be used to
+fetch schema from jio storage. Example:
+
+```javascript
+rJS(window)
+  .allowPublicAcquisition("downloadJSON", function (arr) {
+    var g = this,
+      url = arr[0],
+      reference,
+      args;
+    //   return g.jio_getAttachment(id, "data", {format: "json"});
+    if (url.startsWith("urn:jio:reference?")) {
+      reference = decodeURIComponent(url.replace("urn:jio:reference?", ""));
+      args = {
+        query: '(portal_type: "JSON Schema") AND ((reference: "' + reference + '"))',
+        limit: [0, 1],
+        select_list: [],
+        sort_on: [["modification_date", "descending"]]
+      };
+      return g.jio_allDocs(args)
+        .push(function (result) {
+          return g.jio_getAttachment(result.data.rows[0].id, "data", {format: "json"});
+        });
+    }
+  })
+``` 
+
 ## What inside
 
 gadget consists of three parts: