Create config.json

By adding this, it will simplify the usage of this model in ctransformers:

```py
from ctransformers import AutoModelForCausalLM

llm = AutoModelForCausalLM.from_pretrained("TheBloke/MPT-7B-GGML")
```

config.json

@@ -0,0 +1,3 @@
+{
+  "model_type": "mpt"
+}