Added README with instructions for gen_diagram

quad/sw/modular_quad_pid/gen_diagram/README.md

+The gen_diagram project can create an image of the controller network. Below outlines the process of how to use it.
+
+Within the `quad/sw/modular_quad_pid` directory:
+
+1.  Copy the `control_algorithm_init` function from `src/control_algorithm.c` into `gen_diagram/generate.c`, replacing the previous implementation.
+2.  Within the `gen_diagram` folder, run `make clean`. Then run `make` to build the new code.
+3. Now execute the program with `./gen_diagram`
+4. If you have graphviz installed, you can run `dot -Tpng network.dot -o network.png` to generate a PNG file of the graph.
+
+Enjoy your new diagram!
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