Document how to check for open permissions

Suggested by @go-compile , thanks!

Installation-linux.md

@@ -73,6 +73,16 @@ There are quite a few files here, but you only need one, matching your operating
 
 So, download this file and extract it wherever you want. It can be in your home directory, in `/opt/dnscrypt-proxy`, or wherever you want, really.
 
+You're going to install a new command and important configuration files on your system. If you're new to Linux, for good hygiene, you can quickly check that the directory you extracted cannot be modified by other system users. Here's one way to do it. Type:
+
+```sh
+sudo -u \#1 touch .
+```
+
+(don't forget the final `.`, and there's a space before)
+
+If the command asks for a password, enter your password. If the command immediately returns without printing an error, it's recommended to check the permissions of this directory and all its parent directories ([tutorial on Linux permissions](https://www.howtogeek.com/437958/how-to-use-the-chmod-command-on-linux/#:~:text=Linux%20file%20permissions%20can%20be,to%20add%20or%20remove%20permissions.)).
+
 It is totally possible to have the executable file in one place, the configuration file in another place, the cache files elsewhere and the log files yet somewhere else.
 
 But if this is the first time you install the software, and you don't have any good reasons to makes things more complicated than they should be, just keep everything in the same directory. At least to start with, and to ensure that everything works as expected.