Clarify CHANGED_FILES and rebuild instructions

courses/README.md

@@ -13,7 +13,23 @@ E.g. /static/default or /static/aplus_manual_master
 - STATIC_CONTENT_HOST: full static file URL (including scheme and host).
 E.g. https://gitmanager.cs.aalto.fi/static/default
 - CHANGED_FILES: files changed since last successful build, separated
-by a newline (\\n). This can be used to optimize build steps.
+by a newline (\\n). If changes couldn't be detected or there were too many,
+the variable is set to `*`. This can be used to optimize build steps. This variable
+contains at most 10 files and is set to `*` if there are more than 10. If your build
+script depends on this variable but you wish to trigger a full rebuild, you have
+the following options:
+	1. Make all build steps dependent on a single file that rarely changes. You
+	can then trigger a full rebuild whenever by changing that file (e.g. add/remove an
+extra new line). For example, making everything dependent on the main build script
+would most likely be a good idea.
+	2. Check the "Set CHANGED_FILES to *" box in the Git Manager UI and trigger the
+build manually.
+	3. Temporarily add `rebuild_all=true` to the GET parameters of the git commit hook
+URL.
+	4. Add a temporary change to the start of your build script that changes the
+`CHANGED_FILES` variable to `*`.
+	5. Temporarily modify your build script to ignore `CHANGED_FILES`.
+	6. Modify 10+ files.
 
 ## Configuration files