Update Historical-postcards-metadata-readme.md

metadata-readme/Historical-postcards-metadata-readme.md

 This Readme offers explanations for all attributes of the metadata csv table to the dataset "Historical Postcards"
 
-| attribute | explanation |     
-| ---- | ---- |     
-| a | b |     
+| attribute | explanation |  frequency |      
+| ---- | ---- | ---- |     
+| akon_id | id (shelfmark) of the document | all |     
+| id | id of data record in KAWAN  | all |    
+| altitude | float for geolocation altitude | some |    
+| building | has literal, if building was tagged in metadata editing | some |    
+| city | has literal, if city was tagged in metadata editing | most |    
+| color | has boolean True for color postcards | all |    
+| comment | has literal for | most |     
+| mountain | has literal, if mountain was tagged in metadata editing | some |    
+| other | has literal for any other tags | some |      
+| photographer | has literal, if photographer was identified. Name usually in the form \{forename surname\}. Not linked to GND | some | 
+| publisher | has literal, if publisher was identified. Name usually in the form \{forename surname\} OR \{surname\}. Not linked to GND | some |     
+| publisher_place | has literal, if publication place was identified. Not linked to Geonames or GND | some |