Added README

Author: vladsrynty

README.md

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+# XBRL file converter from XML to JSON format
+
+Convert XBRL-based financial/business document from XML to JSON format, according to XBRL Consortium recommendations ([xbrl-json-CR-2021-02-03](https://www.xbrl.org/Specification/xbrl-json/CR-2021-02-03/xbrl-json-CR-2021-02-03.html)).
+
+## Quickstart example
+
+Here is an simple command line tool to convert xbrl file to json
+
+```php
+use Datalane\XBRL\Instance;
+use Datalane\JSONFile;
+
+require __DIR__ . '/../vendor/autoload.php';
+
+$x = new SimpleXMLElement(file_get_contents($argv[1]));
+
+
+$inst = new Instance($x);
+echo JSONFile::convertInstance($inst);
+```