Create creating-a-new-service.md (#24)

docs/creating-a-new-service.md

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+# Creating a new service
+
+We will be using [this PR](https://github.com/microsoft/component-detection/pull/12) following `EnvironmentVariableService` as a model for adding a new service.
+The following steps are distilled and organized from that PR.
+
+1. Create your new service interface in `src/Microsoft.ComponentDetection.Contracts/IMyNewService.cs`
+2. Create your new service implementation in `src/Microsoft.ComponentDetection.Common/MyNewService.cs` implementing and exporting `IMyNewService`.
+
+```c#
+using System;
+using System.Composition;
+using Microsoft.ComponentDetection.Contracts;
+
+namespace Microsoft.ComponentDetection.Common
+{
+    [Export(typeof(IMyNewService))]
+    public class MyNewService : IMyNewService
+    {
+        ...
+    }
+}
+```
+
+3. Add your new service to `src/Microsoft.ComponentDetection.Contracts/IDetectorDependencies.cs`
+
+```c#
+namespace Microsoft.ComponentDetection.Contracts
+{
+    public interface IDetectorDependencies
+    {
+        ...
+        IMyNewService MyNewService { get; set; }
+    }
+}
+```
+
+4. Add your new service to `src/Microsoft.ComponentDetection.Common/DetectorDependencies.cs`
+
+```c#
+namespace Microsoft.ComponentDetection.Common
+{
+    [Export(typeof(IDetectorDependencies))]
+    public class DetectorDependencies : IDetectorDependencies
+    {
+        ...
+        [Import]
+        public IMyNewService MyNewService { get; set; }
+    }
+}
+```
+
+5. Add your new service to `src/Microsoft.ComponentDetection.Contracts/Internal/InjectionParameters.cs`
+
+```c#
+namespace Microsoft.ComponentDetection.Contracts.Internal
+{
+    internal class InjectionParameters
+    {
+        internal InjectionParameters(IDetectorDependencies detectorDependencies)
+        {
+            ...
+            myNewServiceStatic = detectorDependencies.MyNewService;
+        }
+    }
+    
+    ...
+    private static IMyNewService myNewServiceStatic;
+    
+    ...
+    [Export(typeof(IMyNewService))]
+    public IMyNewService MyNewService => myNewServiceStatic;
+}
+```