README.md: Update dependencies list

- jq is required by usergen.
- Despite being part of a POSIX.1-2008 environment, m4 is not provided
by Debian or Ubuntu by default.

README.md

@@ -59,13 +59,13 @@ to `slcl`. If required, encryption should be done before uploading e.g.: using
 #### Mandatory packages
 
 ```sh
-sudo apt install build-essential libcjson-dev libssl-dev
+sudo apt install build-essential libcjson-dev libssl-dev m4 jq
 ```
 
 #### Optional packages
 
 ```sh
-sudo apt install cmake xxd jq
+sudo apt install cmake
 ```
 
 ## How to use