Clarify OSS definition for vendors

This came up in DMs regarding Grafana, who have both an open source product and a closed source cloud offering.

content/en/ecosystem/vendors.md

@@ -12,6 +12,11 @@ Some organizations provide a [distribution](/ecosystem/distributions/) (of
 customized OpenTelemetry components), that provides additional capabilities or
 for improved ease of use.
 
+Open Source (OSS) refers to a vendor who has an observability product
+that is [open source](https://opensource.org/osd). The vendor may still have
+other products that are closed source, such as a SaaS offering that
+hosts an open source product for their customers.
+
 {{% ecosystem/vendor-table %}}
 
 ## Add your organization