Created an readme for github

Just some temporary text to link users into simplesamlphp.org for download. Feel free to add more info later on.

README.md

+SimpleSAMLphp
+=============
+
+This is the official repository of the SimpleSAMLphp software.
+
+* [SimpleSAMLphp homepage](https://simplesamlphp.org)
+* [SimpleSAMLphp Downloads](https://simplesamlphp.org/download)