add comment on expirations data

README.md

@@ -57,7 +57,7 @@ Important detail:
 - the blinded course columns are ordered by blinded course index, not by the original source column order
 - this is intentional and helps avoid leaking the original course-name ordering from the source system
 
-### `published_expirations.csv`
+### `expirations.csv`
 
 Columns:
 
@@ -66,7 +66,11 @@ Columns:
 - `course_blinded_index`: 0-based blinded course identifier
 - `our_course`: source flag indicating whether the record was associated with the organization’s own course catalog
 
-### `published_orders.csv`
+If a student purchased our course and then completed that course, a `expirations.csv` record will exist for that student/course. The `expired_date` will be a certain number of months after the date they completed the course, and this depends on the course. Such records will have `our_course = 1`.
+
+Students may also self-report expirations data. They may indicate `our_course = 0` indicating a competitor's brand course, or they may indicate `our_course = 1` to say this course was purchased with us.
+
+### `orders.csv`
 
 Columns: