Add a missing step to the gpt4all instructions (#690)

`migrate-ggml-2023-03-30-pr613.py` is needed to get gpt4all running.

Author: ThatcherC

README.md

@@ -232,13 +232,15 @@ cadaver, cauliflower, cabbage (vegetable), catalpa (tree) and Cailleach.
 
 - Obtain the `gpt4all-lora-quantized.bin` model
 - It is distributed in the old `ggml` format which is now obsoleted
-- You have to convert it to the new format using [./convert-gpt4all-to-ggml.py](./convert-gpt4all-to-ggml.py):
+- You have to convert it to the new format using [./convert-gpt4all-to-ggml.py](./convert-gpt4all-to-ggml.py). You may also need to
+convert the model from the old format to the new format with [./migrate-ggml-2023-03-30-pr613.py](./migrate-ggml-2023-03-30-pr613.py):
 
   ```bash
   python3 convert-gpt4all-to-ggml.py models/gpt4all-7B/gpt4all-lora-quantized.bin ./models/tokenizer.model 
+  python3 migrate-ggml-2023-03-30-pr613.py models/gpt4all-7B/gpt4all-lora-quantized.bin models/gpt4all-7B/gpt4all-lora-quantized-new.bin
   ```
 
-- You can now use the newly generated `gpt4all-lora-quantized.bin` model in exactly the same way as all other models
+- You can now use the newly generated `gpt4all-lora-quantized-new.bin` model in exactly the same way as all other models
 - The original model is saved in the same folder with a suffix `.orig`
 
 ### Obtaining and verifying the Facebook LLaMA original model and Stanford Alpaca model data