Update README.md

README.md

@@ -9,7 +9,7 @@ Extract it and open a terminal in the extracted folder.
 Configure the installation.
 The `--with-openssl-rpath` is needed from Python 3.9 onwards, else pip will say it's missing ssl.
 ```sh
-sudo apt install -y libssl-dev
+sudo apt install -y libssl-dev libffi-dev
 ./configure --enable-optimizations --with-openssl-rpath=/usr/include/x86_64-linux-gnu
 ```