Link the Canvas Roll Call docs in two README mentions

Replace the FIXME placeholder in the opening with the Instructure
Community article that describes the Roll Call (Attendance) LTI tool,
and link the same article from the Requirements bullet so someone
scanning to confirm they have what they need can click through.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -13,7 +13,8 @@ weekly [CSSS seminar series](https://csss.uw.edu/seminars) at the
 a set number of those seminars to get credit.
 
 The script reads the weekly attendance export that the [Canvas Roll
-Call](FIXME) tool emails you, cross-references it against the live Canvas
+Call](https://community.instructure.com/en/kb/articles/662770-what-is-the-roll-call-attendance-tool)
+tool emails you, cross-references it against the live Canvas
 enrollment, and sends warning DMs through the Canvas Conversations API.
 A small Python script writes a short note to each student—scoped to
 the course—in the same language you would have written yourself.
@@ -50,8 +51,9 @@ quarter, annotated with the warning dates from the contact log.
   library, which appeared in 3.11).
 - The `requests` library (`pip install requests` or `apt install
   python3-requests`). Everything else is in the stdlib.
-- A Canvas instance with the Roll Call (Attendance) LTI tool and
-  personal API access enabled for you as the instructor.
+- A Canvas instance with the
+  [Roll Call (Attendance) LTI tool](https://community.instructure.com/en/kb/articles/662770-what-is-the-roll-call-attendance-tool)
+  and personal API access enabled for you as the instructor.
 
 ## Setup