Add Fig as an installation method to the README

[ibayramli]

Check list

• [x] I have performed a self-review of my code
• [x] I have commented my code in hard-to-understand areas
• [x] I have made corresponding changes to the documentation

Description

The Fig Plugin Store supports 1-click install for 400+ shell plugins. We have over 100k users, thousands of whom manage their shell configuration with Fig.

forgit is already listed in the store so we'd love to have it listed as a download method on your readme.

Thanks so much and please let me know if you have any questions!

Type of change

• [ ] Bug fix
• [ ] New feature
• [ ] Refactor
• [ ] Breaking change
• [x] Documentation change

Test environment

• Shell
  • [x] bash
  • [x] zsh
  • [x] fish
• OS
  • [ ] Linux
  • [x] Mac OS X
  • [ ] Windows
  • [ ] Others:

[wfxr]

LGTM. @ibayramli2001 Thanks you!