Update readme

README.md

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 # rate music
 This is a simple tool to rate music on your device by people.
 It can be used at parties to rate your playlist for further analysis.
+
+## Compile
+Use `cargo build -r` to build a release executable.
+
+## Usage
+At first run you will be prompted to add a new user.
+This can be achieved as follows:
+```sh
+# Add a new user
+./rate_music [username]
+
+# Add a new user with a specific ID
+./rate_music [username] [ID]
+```
+
+### Exit the program
+To avoid exiting the program by mistake you have to press `ctrl + m` to leave the prompt.