Clarify sourcing tmux.conf

Avoid any confusion with starting tmux with the `-f` option.

Fixes https://github.com/tmux-plugins/tpm/issues/89

README.md

@@ -30,7 +30,7 @@ Put this at the bottom of `.tmux.conf`:
 
 Reload TMUX environment so TPM is sourced:
 
-    # type this in terminal
+    # type this in terminal if tmux is already running
     $ tmux source ~/.tmux.conf
 
 That's it!