Add autoGPTQ installation instructions

[webbigdata-jp]

I noticed that the installation of gptq sometimes fails.
If you do not manually install gekko, the autoGPTQ installation may fail.
Even if you modify setup.py, it will fail because autoGPTQ installation is executed first.

You can check the behavior below.
Stability_AI_lm_evaluation_harness_gptq_example

[webbigdata-jp]

@mkshing It's just a README fix, so I'd be happy to do a quick check and merge!

[mkshing]

@webbigdata-jp thank you!!