fixup! Add SOP for schema changes (DataBiosphere/azul#2884)

Removed schema change embargo from spec and added reference to SOP.

Author: hannes-ucsc

docs/dcp2_system_design.rst

@@ -40,10 +40,13 @@ projects into the DCP/2), an adapter for processing analysis (meta)data
 from Terra workspaces, and adapters for high-level matrix data from a
 range of sources.
 
-All metadata is in JSON format and complies with the HCA Metadata
-Schema. Aside from minor schema changes that were necessary for
-processing the staging areas, the evolution of the schema is currently
-on hold.
+All metadata is in JSON format and complies with the `HCA Metadata Schema`_. 
+Changes to that schema are made according to standard `DCP/2 operating
+procedures`_.
+
+.. _HCA Metadata Schema: https://github.com/HumanCellAtlas/metadata-schema
+
+.. _DCP/2 operating procedures: dcp2_operating_procedures.rst
 
 The DCP/2 only contains public (meta)data (not controlled access).