Update documentation clarification and error message for new release.

As @ivelsko pointed out on mattermost, the README should state that the tunnel is only necessary when running locally and not via the servers.

sidora.core/README.md

Lines 51 to 58 in 2d6c8dd

    
           4. Establish an ssh tunnel to the pandora database server with 
        
               ```bash 
        
               ssh -L 10001:pandora.eva.mpg.de:3306 <your username>@daghead1 
        
               ``` 
        
               > You must make a new tunnel each time you want to connect (e.g. after you log out or reboot your machine)

Additionally, the error message printed out by get_pandora_connection() is outdated, as it describes the old credentials file format.

sidora.core/R/utils_get_pandora_connection.R

Lines 15 to 17 in 2d6c8dd

    
           "[sidora.core] error: can't find .credentials file. Please create one ", 
        
           "containing three lines:",  
        
           "the database host, the username, the password."

	4. Establish an ssh tunnel to the pandora database server with

	```bash
	ssh -L 10001:pandora.eva.mpg.de:3306 <your username>@daghead1
	```

	> You must make a new tunnel each time you want to connect (e.g. after you log out or reboot your machine)

	"[sidora.core] error: can't find .credentials file. Please create one ",
	"containing three lines:",
	"the database host, the username, the password."