Reserve x-sap- namespace for SAP extensions (OAI#3151)

Author: pavelkornev

registries/_namespace/sap.md

@@ -0,0 +1,21 @@
+---
+owner: ralfhandl, pavelkornev
+issue: 
+description: Extensions created and used by SAP
+layout: default
+---
+
+# <a href="..">{{ page.collection }}</a>
+
+## {{ page.slug }} - {{ page.description }}
+
+
+The `x-{{page.slug}}-` prefix is reserved for extensions created by SAP. These extensions are available for use by anyone.
+
+The official list of supported SAP extensions can be found in [SAP/openapi-specification](https://github.com/SAP/openapi-specification) repository.
+
+{% if page.issue %}
+### GitHub Issue
+
+* [#{{ page.issue }}](https://github.com/OAI/OpenAPI-Specification/issues/{{ page.issue }})
+{% endif %}