Add SnapshotTestingHEIC to plugins list (pointfreeco#561)

* Add SnapshotTestingHEIC to plugins list

* Fix README

Author: alexey1312

README.md

@@ -196,6 +196,8 @@ targets: [
 
   - [swift-snapshot-testing-stitch](https://github.com/Sherlouk/swift-snapshot-testing-stitch/) adds the ability to stitch multiple UIView's or UIViewController's together in a single test.
 
+  - [SnapshotTestingHEIC](https://github.com/alexey1312/SnapshotTestingHEIC) adds image support using the HEIC storage format which reduces file sizes in comparison to PNG.
+  
 Have you written your own SnapshotTesting plug-in? [Add it here](https://github.com/pointfreeco/swift-snapshot-testing/edit/master/README.md) and submit a pull request!
 
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