Update README of template with replacement instructions

Author: s0

courses/template/README.md

@@ -3,4 +3,17 @@
 Copy this entire directory,
 and replace the following:
 
-* TODO
+* Replace `<owner>`, `<repo>` and `<pkg>` in the `image` property in
+  [`action.yml`](action.yml) to reference the correct repository
+  where the docker image will be published,
+  and with a package name of your choice.
+  (For courses in this repository,
+  we use the convention of taking the course path,
+  and replacing slashes with dashes,
+  e.g. `courses/cpp/ctf-segv` becomes `courses-cpp-ctf-segv`)
+* Replace the zip file URL in [`image/Dockerfile`](image/Dockerfile)
+  to point to the CodeQL database that will be used in your course.
+
+After this,
+update [`answers/`](answers) and [`image/config/`](image/config)
+to add your model answers and expected query results as appropriate.