update README.md (#968)

added a reminder and a python script in starter code to request access
and log in to huggingface before running mistral7b samples.

README.md

@@ -66,6 +66,12 @@ is to ensure that there is a well-defined interface between their output and
 user-defined code. **Outlines** provides ways to control the generation of
 language models to make their output more predictable.
 
+Before using mistral models, request access on huggingface [here](https://huggingface.co/mistralai/Mistral-7B-Instruct-v0.2).
+``` python
+# login to access mistral model
+from huggingface_hub import login
+login()
+```
 ### Multiple choices
 
 You can reduce the completion to a choice between multiple possibilities: