Adding README for Microsoft.Extensions.EnumStrings (#4621)

* Adding README for Microsoft.Extensions.EnumStrings

* Update src/Libraries/Microsoft.Extensions.EnumStrings/README.md

Co-authored-by: Sébastien Ros <sebastienros@gmail.com>

* Update src/Libraries/Microsoft.Extensions.EnumStrings/README.md

Co-authored-by: Chris Ross <Tratcher@Outlook.com>

---------

Co-authored-by: Sébastien Ros <sebastienros@gmail.com>
Co-authored-by: Chris Ross <Tratcher@Outlook.com>

src/Libraries/Microsoft.Extensions.EnumStrings/README.md

@@ -1 +1,80 @@
-README
+# Microsoft.Extensions.EnumStrings
+
+It's often necessary to convert a C# enum value to a string. The standard `ToString()` method is normally used for this, but that can not be the best in terms of allocations or performance. This package introduces a source generator that is triggered by an attribute which produces an extension method for each `enum` type annotated with the attribute which can efficiently convert an enumeration value to a string.
+
+## Install the package
+
+From the command-line:
+
+```dotnetcli
+dotnet add package Microsoft.Extensions.EnumStrings
+```
+
+Or directly in the C# project file:
+
+```xml
+<ItemGroup>
+  <PackageReference Include="Microsoft.Extensions.EnumStrings" Version="[CURRENTVERSION]" />
+</ItemGroup>
+```
+
+## Usage Example
+
+### Adding the attribute to an enum
+
+To use this feature, you can annotate an enum value with the `EnumStringAttribute` attribute. For example:
+
+```csharp
+using Microsoft.Extensions.EnumStrings;
+
+[EnumStrings]
+public enum Color
+{
+    Red,
+    Green,
+    Blue
+}
+```
+
+As a result, the code generator produces a method with the following signature:
+
+```csharp
+internal static class ColorExtensions
+{
+    public static string ToInvariantString(this Color value);
+}
+```
+
+Which you can later use as follows:
+
+```csharp
+{
+    var color = Color.Red;
+
+    var s1 = color.ToString(); // <- This line will cause an allocation and won't cache the result.
+    var s2 = color.ToInvariantString(); // <- This call is more performant and won't cause a new allocation.
+}
+```
+
+Both calls will return the same string, but the second one will be more efficient.
+
+### Using the generator with an enum from a different assembly
+
+You can also use this feature to generate efficient `ToInvariantString` extension methods on enums that are not part of your code. You can do this by applying the `EnumStrings` attribute at the assembly level, and passing in the type of the enum you want to generate the extension method for. For example:
+
+```csharp
+[assembly: EnumStrings(typeof(Color))]
+```
+
+### Controlling the generated code
+
+The `EnumStringsAttribute` has a few optional properties to let you control the generated class and method. Here are the things that can be controlled:
+
+- `ExtensionNamespace`: Lets you control the namespace into which the extension class is generated. By default, the class will be generated in the same namespace as the enum.
+- `ExtensionClassName`: Lets you control the name of the extension class. This defaults to the name of the enum with the `Extensions` suffix appended to it. For example, if the enum is named `Color`, the extension class will be named `ColorExtensions`.
+- `ExtensionMethodName`: Lets you control the name of the method that is generated. This defaults to `ToInvariantString`.
+- `ExtensionClassModifiers`: Lets you control the access modifiers of the generated class. This defaults to `internal static`. Another common value would be `public static`.
+
+## Feedback & Contributing
+
+We welcome feedback and contributions in [our GitHub repo](https://github.com/dotnet/extensions).