update readme

README.md

@@ -1,15 +1,37 @@
 # Pure PHP RRDtool file reader
 
-> Because on windows php ext-rrd can not read rrd files created on unix.
+> Because on windows php ext-rrd can not read rrd files created on unix and vice versa.
+> See https://github.com/oetiker/rrdtool-1.x/issues/759
 
-**Only meant to export data out of rrd file. Not support for consolidation function etc**
+This library supports reading:
+* rrds created on 64bit linux from 64bit Windows
+* rrds created on 64bit Windows from 64bit Linux
+
+**Only meant to export/dump data out of rrd file. No support for consolidation function etc**
 
 Original repository: http://javascriptrrd.sourceforge.net/
 
 ## Example
 
 ```
+use RrdPhpReader\RrdReader;
+
 $reader = RrdReader::createFromPath('path/to/my_rrd.rrd');
 
 $data = $reader->setDs('value')->setRraIndex(0)->getAsArray();
-```
+
+```
+
+`$data` structure would be
+```
+[
+    'value' => [ <--- datasource as first level keys
+        0 => [   <--- rra index as key
+            1521054891 => 6,  <--- timestamp => value
+            1521054892 => 7,
+            1521054893 => 8,
+            1521054894 => 9,
+        ]
+    ]
+];
+```