Add migration guide for QRCoder v2 with new features and breaking changes

Author: Shane32

docs/migrations/migration2.md

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+# Migration Guide: QRCoder v2
+
+This guide helps you migrate from QRCoder v1.x to v2.x.
+
+## New Features
+
+_Coming soon - this section will be populated as new features are added to v2._
+
+## Breaking Changes
+
+### Minimum framework version is now .NET Standard 2.0
+
+Upgrade your project to one of the following frameworks if required:
+
+- .NET Framework 4.6.2 or higher
+- .NET Core 2.0 or higher
+- .NET 5 or higher