Update CONTRIBUTING.md to add guidance on supporting for .NET versions (open-telemetry#1293)

Author: vishweshbankwar

CONTRIBUTING.md

@@ -258,3 +258,17 @@ the main branch. The workflow file should be named as
 [![NuGet version badge](https://img.shields.io/nuget/v/{your_package_name})](https://www.nuget.org/packages/{your_package_name})
 [![NuGet download count badge](https://img.shields.io/nuget/dt/{your_package_name})](https://www.nuget.org/packages/{your_package_name})
 ```
+
+### Guidance for components on supporting target frameworks
+
+* SHOULD support the [.NET
+  versions](https://github.com/open-telemetry/opentelemetry-dotnet/tree/main#supported-net-versions)
+  which are supported by the OpenTelemetry main repo packages.
+* MUST document the target frameworks supported in the main README.md file.
+* MUST NOT support out-of-support .NET runtimes (e.g.: `.NET Framework 4.5.2`,
+  `.NET Core 2.1` etc). CI checks in this repository will not be run against out
+  of support versions.
+* Whenever a .NET version reaches end of support, components MUST drop support
+  for it as well. Note: This change does not require major version bump. For
+  reference see
+  [this](https://github.com/open-telemetry/opentelemetry-dotnet/pull/3351).