Fix FakeTimeProvider namespace, add readme

src/Libraries/Microsoft.Extensions.TimeProvider.Testing/README.md

@@ -1 +1,47 @@
-README
+# Microsoft.Extensions.TimeProvider.Testing
+
+Provides a `FakeTimeProvider` for testing components that depend on `System.TimeProvider`.
+
+## Install the package
+
+From the command-line:
+
+```dotnetcli
+dotnet add package Microsoft.Extensions.TimeProvider.Testing
+```
+
+Or directly in the C# project file:
+
+```xml
+<ItemGroup>
+  <PackageReference Include="Microsoft.Extensions.TimeProvider.Testing" Version="[CURRENTVERSION]" />
+</ItemGroup>
+```
+
+## Usage Example
+
+`FakeTimeProvider` can be used to manually adjust time to test time dependent components in a deterministic way.
+
+`FakeTimeProvider` derives from [TimeProvider](https://learn.microsoft.com/dotnet/api/system.timeprovider) and adds the following APIs:
+
+```csharp
+public FakeTimeProvider(DateTimeOffset startDateTime)
+public DateTimeOffset Start { get; }
+public TimeSpan AutoAdvanceAmount { get; set; }
+public void SetUtcNow(DateTimeOffset value)
+public void Advance(TimeSpan delta)
+public void SetLocalTimeZone(TimeZoneInfo localTimeZone)
+```
+
+These can be used as follows:
+
+```csharp
+var timeProvider = new FakeTimeProvider();
+var myComponent = new MyComponent(timeProvider);
+timeProvider.Advance(TimeSpan.FromSeconds(5));
+myComponent.CheckState();
+```
+
+## Feedback & Contributing
+
+We welcome feedback and contributions in [our GitHub repo](https://github.com/dotnet/extensions).