Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -4,3 +4,23 @@
 Please send pull requests against the development branch instead of master. The
 master branch is reserved to contain only the latest official public release of 
 the framework.
+
+
+## Use C# 4.0
+
+If possible, when contributing code to the framework, please avoid using C# language features from above C# 4.0. There are at least two reasons for this restriction:
+
+ 1. **Mono:** To keep compatibility with the most widespread versions of Mono (4.x). If you take a look at our Travis-CI builds, you might see that the builds are actually done and run using Mono, so using any language feature that is not supported there will cause a failing build;
+ 1. **Unity:** Some language features might not be accessible when targeting .NET 3.5, which for a long time has been the only .NET Framework version that could be run from inside [Unity](https://unity3d.com).
+
+More specifically, when submitting pull-requests, please avoid using:
+
+ * The nameof(.) operator;
+ * Expression-bodied members;
+ * Null-conditional operators
+ * string interpolation
+ * async/await *
+ * value tuples
+
+_\* Unless you can guard those sections using conditional compilation clauses (i.e. ```#if NET35```) and either exclude those code sections from the .NET 3.5 / Mono 4.0 builds or provide specific (possibly non-optimal) implementations for those platforms. Please take a look at the Accord.Compat namespace for help in filling missing functionality from higher platform versions if you run into any of those cases._
+