Add all available configuration options in README

[mksonkar]

I know the options are mentioned in :help gruvbox-material.txt.
But it will be really useful if anyone new checks the page, and just being able to copy all available options to tweak theme options from the README page. If the user wants to read what they do then they can refer to the documentation. Many other themes provide the full configuration options on the README and it will help with making it more popular and accessible to new users.

[antoineco]

Besides the slightly improved rendering while navigating the docs in a web browser, I don't see a lot of value doing this. It's a lot of effort for essentially just duplicating information:

• To make it viable, we need tooling for keeping both versions in sync. There are projects that generate Vim docs from markdown but not the other way around, so the initial conversion needs to be manual.
• To avoid drifts in the future, we need to
  • enforce Git hooks.
  • set up GitHub Actions to verify that hooks were executed, or execute them on behalf of contributors after changes are merged.

To me, the root of the problem is a lack of web-based Vimdoc renderer. The Vim and Neovim projects generate and host their own HTML, which is relevant to them given the size of their documentation, but not to a colorscheme with very few exposed switches.

It is true that some colorschemes have their options enumerated in their README, but the majority are written in Lua and use a much more complex table-based configuration schema than Gruvbox Material.

If you have a end-to-end solution for this, PR welcome!