Improve the documentation

README.md

@@ -15,6 +15,8 @@ Its only dependency is Python >= 3.7 (use `python3 -V` to check your version).
 First of all, you need to extract the value of the `connect.sid` browser cookie for the
 particular CodiMD instance that you use (e.g., `md.inf.tu-dresden.de`).
 
+This procedure is necessary because the CodiMD login is handled externally by Keycloak.
+
 **WARNING**: the value of the cookie is your session ID and as such, **should be treated like a
 password**. Don't share it with others!
 
@@ -34,6 +36,10 @@ Clone this repository to a path of your liking and just execute `./codimd_export
 shell. The script will ask for your session id and download the notes to the relative path
 `./codimd-documents`.
 
+Please note that your CodiMD history might reference already deleted notes or notes you no
+longer have access to. The URLs of these inaccessible notes are listed as part of the
+script's output.
+
 
 ### Example