Merge pull request #1421 from nunit/CharliePoole-patch-1

Create platform-support.md

Author: CharliePoole

platform-support.md

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+# Platform Support Lifecycle
+
+"Platform Support" for this project includes three somewhat different things:
+1. Test assembly target platforms, that is, the target platforms for which tests may be written.
+2. The target platforms under which those tests will actually run, which is equivalent to the list of agents we provide.
+3. The minimum platforms required to execute the runner and engine themselves, without regard to the tests being run.
+
+## Test Assembly Target Runtimes
+
+We currently support execution of tests written to target any version of the .NET Framework >= 2.0 and any version
+of .NET Core >= 3.1, including .NET 5.0 and higher. We will continue to support tests targeting runtimes which are
+out of support from Microsoft's perspective, so long as we are able to do so without significant additional effort
+and without security risk. If no agent is available for a runtime, the tests will be run on the closest higher 
+runtime available.
+
+## Agents Provided
+
+We currently supply a fairly large selection of agents with the console runner:
+* .NET Framework 2.0
+* .NET Framework 4.6.2
+* ,NET Core 3.1
+* .NET 5.0
+* .NET 6.0
+* .NET 7.0
+* .NET 8.0 (coming in version 3.18.0)
+
+As a general policy, we will continue to provide agents for any Microsoft runtime for at least six months after its
+official end of life. This is intended to support continued testing of legacy applications while users are in the 
+process of upgrade. However, agents for runtimes which have been declared a security risk may be removed immediately.
+
+Based on that policy and the planned end-of-life dates for runtimes, we expect to retire agents on or after the
+dates listed in the following table. 
+
+| Runtime              | Microsoft<br>End of Support | Agent Retirement | Notes |
+| -------------------- | --------------- | --------------------- | --- |
+| .NET Framework 2.0   | July, 2011      | July, 2024            | Will be removed in version 3.18.0
+| .NET Framework 4.6.2 | January, 2027   | after July, 2027      | May be upgraded to 4.8.1 before retirement date |
+| .NET Core 3.1        | December, 2022  | after December, 2024  | 
+| .NET 5.0             | May, 2022       | July, 2024            | Will be removed in version 3.18.0
+| .NET 6.0             | November, 2024  | after May, 2025       |
+| .NET 7.0             | May, 2024       | after November, 2024  |
+| .NET 8.0             | November, 2027  | after May, 2027       |
+
+## Required Runtimes
+
+The console runner and engine target .NET Framework 4.6.2, so that runtime or greater is required to execute any tests at all
+without loss of features. In individual cases, it may be possible to run using a lesser version of .NET 4.x. It's possible
+that we may require a higher level of the framework in a future 3.x release.
+