Merge pull request umbraco#4071 from umbraco/Nullable-reference-types

v10: Created new article for Nullable reference types

Implementation/Nullable-Reference-Types/index.md

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+---
+versionFrom: 10.0.0
+Meta.Title: Nullable Reference Types
+Meta.Description: In this article we describe what Nullable reference types is.
+---
+
+# Nullable Reference Types
+
+Starting from Umbraco 10 Nullable Reference Typs is enabled by default in Umbraco.
+
+Nullable reference types is a group of features introduced in C# 8.0 that can be used to minimize the likelihood that your code causes the runtime to throw `System.NullReferenceException`.
+
+Nullable reference types includes three features that help you avoid these exceptions, including the ability to explicitly mark a reference type as nullable:
+
+- Improved static flow analysis that determines if a variable may be null before dereferencing it.
+- Attributes that annotate APIs so that the flow analysis determines null-state.
+- Variable annotations that developers use to explicitly declare the intended null-state for a variable.
+
+To learn more about Nullable Reference Types, make sure to check out the [Microsoft Documentation](https://docs.microsoft.com/en-us/dotnet/csharp/nullable-references)
+