Update readme with build environment

[mikesol]

Hi,

First and foremost, thank you very much for this helpful repo.
I was wondering if you could update the README with a bit more info about your build environment? I am building on Ubuntu in Windows, but when I try to start the chromedriver on AWS Lambda, the function returns the following error:

Service chromedriver unexpectedly exited. Status code was: 127

It looks like this most often has to do with a malformed binary or missing libraries, which I'm guessing may have to do with where I'm building the layer. If you have time to include recommendations for where to build the layer, that may help.

Thanks!