Add a convention file for the extension id

conventions/extension-id.md

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+# Extension ID
+
+## Naming
+
+The extension shall be named in the form: `clap.$NAME/$REV`.
+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.
+
+## Draft
+
+An extension is considered a draft extension if it is in the [draft](../include/clap/ext/draft/) folder.
+
+When the extension is migrating from draft to stable, its extension ID must not change.
+
+All extensions must go first though a draft phase.
+
+## History
+
+Before this document was written, existing extension didn't honor those rules.
+Some stable extension include the string `draft` in their ID.
+We decided to keep those in order to maintain the binary compatibility.