add readme

Signed-off-by: Vladimir Petrusevici <[email protected]>

Author: vpetrusevici

src/OpenFeature.Contrib.Providers.Flagsmith/README.md

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+# Flagsmith Feature Flag .NET Provider
+
+The Flagsmith Flag provider allows you to connect to your Flagsmith instance.  
+
+# .Net SDK usage
+
+## Install dependencies
+
+The first things we will do is install the **Open Feature SDK** and the **Flagsmith Feature Flag provider**.
+
+### .NET Cli
+```shell
+dotnet add package OpenFeature.Contrib.Providers.Flagsmith
+```
+### Package Manager
+
+```shell
+NuGet\Install-Package OpenFeature.Contrib.Providers.Flagsmith
+```
+### Package Reference
+
+```xml
+<PackageReference Include="OpenFeature.Contrib.Providers.Flagsmith" />
+```
+### Packet cli
+
+```shell
+paket add OpenFeature.Contrib.Providers.Flagsmith
+```
+
+### Cake
+
+```shell
+// Install OpenFeature.Contrib.Providers.Flagsmith as a Cake Addin
+#addin nuget:?package=OpenFeature.Contrib.Providers.Flagsmith
+
+// Install OpenFeature.Contrib.Providers.Flagsmith as a Cake Tool
+#tool nuget:?package=OpenFeature.Contrib.Providers.Flagsmith
+```
+
+## Using the Flagsmith Provider with the OpenFeature SDK
+
+To create a Flagmith provider you should define provider and Flagsmith settings.
+
+```csharp
+using OpenFeature.Contrib.Providers.Flagd;
+
+namespace OpenFeatureTestApp
+{
+    class Hello {
+        static void Main(string[] args) {
+
+            // Additional configs for provider
+            var providerConfig = new FlagsmithProviderConfiguration();
+
+            //Flagsmith client configuration
+            var flagsmithConfig = new FlagsmithConfiguration
+            {
+                ApiUrl = "https://edge.api.flagsmith.com/api/v1/",
+                EnvironmentKey = string.Empty,
+                EnableClientSideEvaluation = false,
+                EnvironmentRefreshIntervalSeconds = 60,
+                EnableAnalytics = false,
+                Retries = 1
+            };
+            var flagsmithProvider = new FlagsmithProvider(providerConfig, flagsmithConfig);\
+
+            // Set the flagsmithProvider as the provider for the OpenFeature SDK
+            OpenFeature.Api.Instance.SetProvider(flagsmithProvider);
+
+            var client = OpenFeature.Api.Instance.GetClient("my-app");
+
+            var val = client.GetBooleanValue("myBoolFlag", false, null);
+
+            // Print the value of the 'myBoolFlag' feature flag
+            System.Console.WriteLine(val.Result.ToString());
+        }
+    }
+}
+```
+
+You also can create Flagsmith provider using ```HttpClient``` or precreated ```FlagsmithClient```
+
+```csharp
+using var httpClient = new HttpClient();
+var flagsmithProvider = new FlagsmithProvider(providerConfig, config, httpClient);
+```
+```csharp
+using var flagsmithClient = new FlagsmithClient(flagsmithOptions);
+var flagsmithProvider = new FlagsmithProvider(providerConfig, flagsmithClient);
+```
+### Configuring the FlagsmithProvider
+
+To configure FlagsmithConfiguration just use [an example](https://github.com/Flagsmith/flagsmith-dotnet-client/tree/main/Example) from Flagsmith GitHub.
+For FlagsmithProviderConfiguration you can configure next parameters using custom implementation or just ```FlagsmithProviderConfiguration```:
+```csharp
+public interface IFlagsmithProviderConfiguration
+{
+    /// <summary>
+    /// Key that will be used as identity for Flagsmith requests.
+    /// </summary>
+    public string TargetingKey { get; }
+
+    /// <summary>
+    /// Determines whether to resolve a feature value as a boolean or use
+    /// the isFeatureEnabled as the flag itself. These values will be false
+    /// and true respectively.
+    /// Default: false
+    /// </summary>
+    public bool UsingBooleanConfigValue { get; }
+}
+```
+
+