docs: updated the README.md with new link of yarn install instructions

[abhishekjakhar]

The route of link https://yarnpkg.com/docs/install does not exist anymore and has been changed to https://yarnpkg.com/getting-started/install, I have updated the new route in the README.md

Opening the current route takes to the NOT FOUND page.

Thank you for contributing to Reach UI! Please fill in this template before submitting your PR to help us process your request more quickly.

• Use a meaningful title for the pull request. Include the name of the package modified.
• Test the change in your own code (Compile and run).
• Add or edit tests to reflect the change (Run with yarn test).
• Add or edit Storybook examples to reflect the change (Run with yarn start).
• Ensure formatting is consistent with the project's Prettier configuration.

This pull request:

• Creates a new package
• Fixes a bug in an existing package
• Adds additional features/functionality to an existing package
• Updates documentation or example code
• Other

If creating a new package:

• Make sure the new package directory contains each of the following, and that their structure/formatting mirrors other related examples in the project:
  • examples directory
  • src directory with an index.tsx entry file
  • At least one example file per feature introduced by the new package
  • Base styles in a style.css file (if needed by the new package)

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit 196cc56:

Sandbox	Source
determined-ganguly-5t1m6	Configuration