Adding extra step to README

README.md

 **Note**: Please remember that it's a prerequisite the Domain A record needs to be pointed to the server's public IP and resolving.
 
 1. Download the `matrix` directory, recommended: `/tmp`
-2. Enter the directory and run the installer, you will need the domain name, e.g.:
+2. Enter the directory and grant exec permissions to run the installer and execute it, you will need the domain name, e.g.:
 
 ```shell
 cd /tmp/matrix
+chmod +x install.sh
 ./install.sh <DOMAIN>
 ```