Add link to VSCode extension

[NathHorrigan]

Summary

This PR adds a link to the README that points users to the VS-Code extension implementation of SVGR I have worked on. I've gotten some positive feedback on the extension so thought would be good to share a link and make it more easily accessible to devs looking into SVGR and how it can fit into their workflow.

Test plan

Screenshot from README:

[vercel]

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🔍 Inspect: https://zeit.co/gregberge/svgr/kzj0qi9gk
✅ Preview: https://svgr-git-fork-nathhorrigan-feature-vs-code-link.gregberge.now.sh

[codecov]

Codecov Report

Merging #434 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #434   +/-   ##
=======================================
  Coverage   86.53%   86.53%           
=======================================
  Files          31       31           
  Lines         557      557           
  Branches      153      153           
=======================================
  Hits          482      482           
  Misses         60       60           
  Partials       15       15           

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[gregberge]

Thanks!