Add v7 migration guide

Author: chawes13

README.md

@@ -50,6 +50,7 @@ Documentation and usage info can be found in [docs.md](docs.md).
 
 - [v5.0.0](migration-guides/v5.0.0.md)
 - [v6.0.0](migration-guides/v6.0.0.md)
+- [v7.0.0](migration-guides/v7.0.0.md)
 
 ## Contribution
 

migration-guides/v7.0.0.md

@@ -0,0 +1,6 @@
+# v7.0.0 Migration Guide
+
+## Breaking changes
+This library no longer supports Internet Explorer (IE). Previous iterations had a polyfill for the [Symbol](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol) primitive, which is fully supported by all major browsers.
+
+If you have an older project that needs to support IE, avoid this version or bring in your own Symbol polyfill.