adding apt update in readme

to prevent:
```
$ sudo apt install quickemu
Reading package lists... Done
Building dependency tree       
Reading state information... Done
E: Unable to locate package quickemu
```

README.md

@@ -91,6 +91,7 @@ install Quickemu and all the dependencies run the following in a terminal:
 
 ```bash
 sudo apt-add-repository ppa:flexiondotorg/quickemu
+sudo apt update
 sudo apt install quickemu
 ```