Add git lfs to readme

[wookiehangover]

When I tried following the readme to start the gradio demo, I got an error due to missing the weights, since they are not downloaded in the initial git checkout. Running git lfs pull fixed my issue. This change adds the git lfs pull step to the readme.

[HarborYuan]

Thanks for pointing out. Usually, if the lfs is enabled (git lfs install), there is no need to git lfs pull again. How about writing git lfs install in the beginning instead?

[wookiehangover]

Done, thanks for the feedback!

[HarborYuan]

Hi @wookiehangover ,

I have made a small modification to your commit. You can try and figure out whether it works well. Please let me know if you have any other questions.

Thanks for your contribution and have a nice day.