Add per entry and per property meta data field.

optimade.rst

@@ -442,6 +442,33 @@ For example, the following query can be sent to API implementations `exmpl1` and
 
 :filter:`filter=_exmpl1_band_gap<2.0 OR _exmpl2_band_gap<2.5`
 
+Per property metadata fields
+----------------------------
+
+Implementations are allowed to specify an OPTIONAL field containing per entry metadata for a property.
+The name of the metadata field consists of the name of the property for which it contains the metadata, suffixed with "_meta".
+For example, when the field is :property:`cartesian_site_positions` the metadata field would be :field:`cartesian_site_positions_meta`.
+
+This metadata field consists of a dictionary which MAY contain database specific fields.
+Fields in this metadata MAY also have a metadata field.
+If an implementation supports the metadata field, it SHOULD return the metadata field whenever the property to which the metadata field belongs is returned.
+
+
+Example:
+
+ .. code:: jsonc
+
+ {
+ "element_ratios":[0.33336, 0.22229, 0.44425],
+ "element_ratios_meta": {
+ "_exmpl_confidence_interval": [[0.33325,0.33347],[0.22190,0.22268],[0.44392,0.44458]],
+ "_exmpl_confidence_interval_meta":{
+ "_exmpl_confidence_level": 0.95
+ }
+ //...
+ }
+
+
 Responses
 =========