Document.

README.md

@@ -1,5 +1,51 @@
 # Subatomix.Testing
+
+[![Build](https://github.com/sharpjs/Subatomix.Testing/workflows/Build/badge.svg)](https://github.com/sharpjs/Subatomix.Testing/actions)
 [![NuGet](https://img.shields.io/nuget/v/Subatomix.Testing.svg)](https://www.nuget.org/packages/Subatomix.Testing)
 [![NuGet](https://img.shields.io/nuget/dt/Subatomix.Testing.svg)](https://www.nuget.org/packages/Subatomix.Testing)
 
-My preferred frameworks for automated testing.
+My preferred frameworks for automated testing in .NET.
+
+See [here](https://github.com/sharpjs/Subatomix.Testing/blob/master/Subatomix.Testing/Subatomix.Testing.csproj)
+for a list of included packages and their versions.
+
+This package also includes the
+[`TestHarnessBase`](https://github.com/sharpjs/Subatomix.Testing/blob/master/Subatomix.Testing/TestHarnessBase.cs)
+class, which aids my preferred technique for setup/teardown code.  Generally, I
+eschew traditional `SetUp` and `TearDown` methods.  Instead, in each test, I
+create an instance of a disposable context class.  Construction is setup, and
+disposal is teardown.  Because `TestContext` means something else already in
+NUnit, I call this pattern *Test Harness* instead.
+
+```csharp
+[Test]
+public void TestSomething()
+{
+    using var h = new TestHarness();
+
+    // rest of test
+}
+
+private class TestHarness : TestHarnessBase
+{
+    // properties for mocks and things
+
+    public TestHarness()
+    {
+        // setup code
+    }
+
+    protected override CleanUp()
+    {
+        // teardown code
+    }
+}
+```
+
+This pattern enables some cool things:
+
+- I can enable the C# 8
+  [nullability checker](https://docs.microsoft.com/en-us/dotnet/csharp/nullable-references)
+  and not have to sprinkle `?` and `!` all over the test code.
+- Tests can run in parallel, since there is no longer any shared state within a test class.
+- Test-support code can be isolated away from the tests themselves.