added encryption key in env

.env.example

@@ -6,6 +6,12 @@ LOG_LEVEL=INFO
 UID=1000
 GID=1000
 
+# The key below is used to en/decrypt secrets, that needs to
+# be send from one service to another (mostly via mqtt).
+# One can generate a new key like this:
+# $ python -c "import cryptography.fernet as c; print(c.Fernet.generate_key().decode())"
+FERNET_ENCRYPTION_SECRET=CKoB---DEFAULT-DUMMY-SECRET---0exKVH0QDLy1B=
+
 ############################################################
 # proxy (nginx)
 #  Proxy comes first because we refer to variables here