Add co-host note

Add a note for hosts on online trainings to appoint fellow trainers as co-hosts.

Author: ndporter

episodes/01-welcome.md

@@ -23,7 +23,7 @@ exercises: 15
 
 
 :::::::::::::::::::::::::::::::::::::::instructor
-In an online training, ensure that captions are turned on and that one trainer claims host. 
+In an online training, ensure that captions are turned on, that one trainer claims host, and that the host adds other trainers as co-hosts. 
 The host key is in the email to trainers from the core team prior to the training. 
 :::::::::::::::::::::::::::::::::::::::::::::::::::::::
 

episodes/08-motivation.md

@@ -22,7 +22,7 @@ exercises: 30
 
 
 :::::::::::::::::::::::::::::::::::::::instructor
-In an online training, ensure that captions are turned on and that one trainer claims host. 
+In an online training, ensure that captions are turned on, that one trainer claims host, and that the host adds other trainers as co-hosts. 
 The host key is in the email to trainers from the core team prior to the training. 
 :::::::::::::::::::::::::::::::::::::::::::::::::::::::
 

episodes/15-carpentries.md

@@ -7,7 +7,7 @@ exercises: 25
 
 
 :::::::::::::::::::::::::::::::::::::::instructor
-In an online training, ensure that captions are turned on and that one trainer claims host. 
+In an online training, ensure that captions are turned on, that one trainer claims host, and that the host adds other trainers as co-hosts. 
 The host key is in the email to trainers from the core team prior to the training. 
 :::::::::::::::::::::::::::::::::::::::::::::::::::::::
 

episodes/20-performance.md

@@ -21,7 +21,7 @@ exercises: 25
 
 
 :::::::::::::::::::::::::::::::::::::::instructor
-In an online training, ensure that captions are turned on and that one trainer claims host. 
+In an online training, ensure that captions are turned on, that one trainer claims host, and that the host adds other trainers as co-hosts. 
 The host key is in the email to trainers from the core team prior to the training. 
 :::::::::::::::::::::::::::::::::::::::::::::::::::::::