Update the extension id convention

conventions/extension-id.md

@@ -7,6 +7,10 @@ Where:
 - `$NAME` is the name of the exension.
 - `$REV` is the revision of the extension. This is an integer that is incremented for each iteration.
 
+For extensions made by third-parties and not officially published by the CLAP project, please use the following form: `$REVERSE_URI.$NAME/$REV`.
+Where:
+- `$REVERSE_URI` would be something like `com.bitwig`.
+
 ## Draft
 
 An extension is considered a draft extension if it is in the [draft](../include/clap/ext/draft/) folder.
@@ -15,6 +19,10 @@ When the extension is migrating from draft to stable, its extension ID must not
 
 All extensions must go first though a draft phase.
 
+## For factory ID
+
+Everything about the extension id simetrically applies to factory id.
+
 ## History
 
 Before this document was written, existing extension didn't honor those rules.