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.