Add migration instructions for photos

migration.md

@@ -54,6 +54,12 @@ This migration typically takes a few minutes.
 
 ## Photos
 
+Make sure the album directories exist somewhere on the server. Then,
+for each album, perform `python manage.py importphotos <album dir>`.
+This needs to be done per album to avoid having to store two copies of all
+albums at the same time. Consider doing several albums at once by using
+your shell to loop over a part of the set (e.g. divide them over directories).
+
 ## Pizzas
 
 ## Thabloids
@@ -63,4 +69,4 @@ This migration typically takes a few minutes.
 Make sure you have valid login credentials for thalia.nu.
 
 To migrate the merchandise execute `python manage.py migratemerchandise` and
-enter the required login information.
\ No newline at end of file
+enter the required login information.