Add Fig as an installation method to the README

[ibayramli]

Standards checklist:

• [x] The PR title is descriptive.
• [x] The PR doesn't replicate another PR which is already open.
• [x] I have read the contribution guide and followed all the instructions.
• [x] The code follows the code style guide detailed in the wiki.
• [x] The code is mine or it's from somewhere with an MIT-compatible license.
• [x] The code is efficient, to the best of my ability, and does not waste computer resources.
• [x] The code is stable and I have tested it myself, to the best of my abilities.

Changes:

• Added Fig as an installation option to the readme, conforming with its style.

Other comments:

• 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. OMZ 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!