Merge pull request #27 from mashukreza/patch-2

Update obligations.html.md

source/documentation/obligations.html.md

@@ -88,9 +88,11 @@ A new obligation is generated on the first day of the period, whether the previo
 18A3 31.10.2018<br/>
 18A4 31.01.2019
 
-Note we have not yet configured annual period keys as annual accounting is not in scope for MVP.
+### Encoding period keys
 
-The period key is the ID code for the period that this obligation belongs to. The format is a string of four alphanumeric characters. Occasionally for special periods, the format includes a # symbol (e.g. #001), so the period key must be percent-encoded, for example 18AD, 18A1, %23001.
+The period key is the ID code for the period that this obligation belongs to. The format is a string of four alphanumeric characters.
+
+The format of the period key is a string of four alphanumeric characters. Occasionally for special periods, the format includes a # symbol (for example #001), so the period key must be percent-encoded, for example 18AD, 18A1, %23001.
 
 ### Example format of returned obligations