Improve comments in the XML catalog

Signed-off-by: Nicolas-Peiffer <[email protected]>

Author: Nicolas-Peiffer

schema/xmlcatalog.xml

@@ -1,11 +1,25 @@
 <?xml version="1.0"?>
+<!--
+  This XML catalog provides mappings for CycloneDX schemas.
+  The catalog maps schema URLs to local XSD files to facilitate schema
+  validation without needing internet access.
+  Namespace: urn:oasis:names:tc:entity:xmlns:xml:catalog
+-->
 <catalog xmlns="urn:oasis:names:tc:entity:xmlns:xml:catalog">
+
+  <!-- SPDX BOM Schema -->
   <uri name="http://cyclonedx.org/schema/spdx" uri="spdx.xsd"/>
+
+  <!-- CycloneDX BOM Schemas -->
   <uri name="http://cyclonedx.org/schema/bom/1.0" uri="bom-1.0.xsd"/>
   <uri name="http://cyclonedx.org/schema/bom/1.1" uri="bom-1.1.xsd"/>
   <uri name="http://cyclonedx.org/schema/bom/1.2" uri="bom-1.2.xsd"/>
   <uri name="http://cyclonedx.org/schema/bom/1.3" uri="bom-1.3.xsd"/>
   <uri name="http://cyclonedx.org/schema/bom/1.4" uri="bom-1.4.xsd"/>
   <uri name="http://cyclonedx.org/schema/bom/1.5" uri="bom-1.5.xsd"/>
   <uri name="http://cyclonedx.org/schema/bom/1.6" uri="bom-1.6.xsd"/>
-</catalog>
+
+  <!-- Placeholder for future schemas, where 1.x is the next CycloneDX Spec Version -->
+  <!-- <uri name="http://cyclonedx.org/schema/bom/1.x" uri="bom-1.x.xsd"/> -->
+
+</catalog>