Updating README.md to account for libfftw system-level dependency on Ubuntu

Author: rvs

README.md

@@ -89,6 +89,7 @@ Start by installing the required packages. For Ubuntu 24.04, the command is:
       nlohmann-json3-dev \
       python3-sphinx python3-sphinx-rtd-theme python3-breathe python3-jsonschema \
       libfmt-dev \
+      libfftw3-dev \
       xxd
 ```