Add ADR for inferring LA from postcode

docs/adr/adr-013-inferring-la-from-postcode.md

@@ -0,0 +1,12 @@
+### ADR - 012: Inferring LA from postcode
+
+We use ONS data to infer local authority from postcode in the property information section.
+The Office for National Statistics (ONS) publishes the National Statistics 
+Postcode Lookup (NSPL) and ONS Postcode Directory (ONSPD) datasets, 
+which may be used to find a local authority district for a postcode when compiling statistics. 
+
+We're using postcodesio API with postcodesio gem.
+Postcodesio uses OS and ONS data which is updated as soon as new data becomes available.
+
+We are not using OS places API due to the lack of data. 
+Closest datapoint to LA in OS places api is ADMINISTRATIVE_AREA which does not always match with local authority.