Add hints on how to handle encrypted files

Write a minimal version of the vault tutorial section of the ansible docs.

README.md

@@ -83,3 +83,9 @@ Be aware that the development branches here are not save and the owner might for
 * Add your SSH-key to the host `ssh-copy-id lasXXX.las.kit.edu`
 * Do the steps described for the self-setup
 * Run `ansible-playbook -K --vault-id @prompt sites.yml` probably with the option `-l lasXXX.las.kit.edu`
+
+## Edit encrypted files
+* You can either use `ansible-vault edit --vault-id @prompt group_vars/all/vault.yml` to edit the file in your editor mentioned in the `$EDITOR` environment variable or
+* you can decrypt the file `ansible-vault decrypt --vault-id @prompt group_vars/all/vault.yml`, edit the file and encrypt it again `ansible-vault encrypt --ask-vault-pass group_vars/all/vault.yml`
+
+The first one is of cause the preferred one, because there is no rist to add a unencrypted file to the repo.