Update installation instructions for non-server (#382)

var/docs/installation.md

@@ -5,6 +5,18 @@ installation instruction in our [README](https://github.com/kevinpapst/kimai2/#i
 
 You need GIT and [Composer](https://getcomposer.org/doc/00-intro.md) on the machine where you want to install Kimai. 
 
+## Installing on a personal computer (instead of a server)
+### 1. No need to use www-data user
+The installation instructions are intended primarily for server applications. 
+If you are installing Kimai 2 on a desktop or laptop computer - even for use in a local network, but where the computer primarily serves as a single user computer - you will avoid permission errors by subsituting ```www-data``` in the relevant commands with your username.
+In particular, ```sudo -u www-data``` is a command which grants the ```www-data``` user temporary administrator/super-user privileges). However, depending on the configuration of your particular computer, you may be able to avoid sudo altogether (your user may already have adequate permissions).
+You can try first leaving ```sudo -u www-data``` altogether in the relevant commands. If you have permission errors, you can substitute it for ```sudo -u username``` in the relevant commands (where username is the username that runs the server - if you don't know, it is likely your own username that you login with).
+
+### 2. Chown & Chmod commands
+Further, chown & chmod commands should be for the username that runs the server instead of ```www-data``` (again, if you don't know, it is likely your own username).
+
+Also note that, depending on where you are installing Kimai 2 and how your computer is configured, you may also receive "operation not permitted" errors when setting file permissions (chown and chmod commands). In that case, prefix them with ```sudo ```.
+
 ## Hosting & 1-click installations
 
 These platforms adopted Kimai 2 to be compatible with their one-click installation systems: