- added a relaxNG test file

example_catalog/schema_catalog.xml

@@ -4,4 +4,5 @@
    <entry schematype="schema" namespace="http://www.lido-schema.org" schemaurl="http://www.lido-schema.org/schema/v1.1/lido-v1.1.xsd" />
    <entry schematype="schema" namespace="http://slubarchiv.slub-dresden.de/rights1" schemaurl="https://slubarchiv.slub-dresden.de/fileadmin/groups/slubsite/slubarchiv/standards/rights/rights1.xsd" />
    <entry schematype="schema" namespace="http://www.loc.gov/standards/alto/ns-v2#" schemaurl="http://www.loc.gov/standards/alto/alto-v2.0.xsd" />
+   <entry schematype="relaxng" namespace="http://www.music-encoding.org/ns/mei" schemaurl="https://raw.githubusercontent.com/music-encoding/music-encoding/v3.0.0/schemata/mei-all.rng" />
 </slubschemacatalog>