docs: add environment variable explanations to README

[LangLangBart]

description

Some explanatory text for environment variables used in this script.

[junegunn]

Thanks.

Comprehensive documentation is a double-edged sword. While it's good to have all the information in one place, it can be intimidating for users who just want to get started. I believe these variables are only relevant to a small number of users because most of them have reasonable defaults. So I'm going to

• move the section to the bottom of the page.
• remove NO_COLOR and GIT_PAGER from the list, because they're not specific to fzf-git.sh
• present default values instead of example values

[LangLangBart]

Thanks for the feedback. Your proposed changes make sense and improve the documentation's clarity.

[junegunn]

Merged, thanks!