docs: Fig installation instructions

[brendanfalk]

Description

Added Fig as an installation method to the readme among other plugin managers.

Motivation and Context

We recently listed fast-syntax-highlighting on Fig Plugin Store and we would love to have Fig listed as an installation method of this plugin. We have over 100k users for whom we think this change will make installation of this plugin easier.

Related Issue(s)

Usage examples

How Has This Been Tested?

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

[vladdoster]

@brendanfalk Thanks so much! Fig is cool.