[fix #451] add why we cannot use forks

README.md

@@ -32,7 +32,11 @@ for your learners to practice in.
     (If you do not have an account, you can quickly create one for free.)
     You must be logged in for the remaining steps to work.
 
-2.  Go to <a href="https://github.com/new/import" target="_blank">GitHub's importer</a>.
+2.  Go to <a href="https://github.com/new/import" target="_blank">GitHub's
+    importer</a>. Note that you do not want to create a fork because any
+    particular user can only have one fork of a repository. Instructors
+    frequently teach more than one workshop and therefore need multiple copies
+    of the repository.
 
 3.  Paste the url of this repo as the old repository to clone:
     <https://github.com/carpentries/workshop-template>.