add some documentation

README.md

@@ -1,3 +1,32 @@
+### Usage
+
+```php
+$path = __DIR__ . '/fixtures/ifdo-test-v2.0.0.json';
+$obj  = Ifdo::fromFile($path);
+
+// print errors to console if document is no valid
+$obj->setDebug(true);
+
+// check if document is valid
+$obj->isValid();
+
+// get list of errors if there are any
+$obj->getErrors();
+
+// get full json as array
+$obj->getJsonData();
+
+// shorthands to safely access info
+$obj->getImageSetHeader();
+$obj->getImageSetItems();
+
+// use strict mode trigger exceptions for invalid files
+$obj = Ifdo::fromString('{"some": "json"}', true);
+
+// get json encoded string
+$obj->toString();
+```
+
 ### Testing
 
 ```bash