Create "Using Elsa Workflow with ABP Framework" article

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+# Using Elsa Workflow with ABP Framework
+
+**Elsa Core** is an open-source workflows library that can be used in any kind of .NET Core application. Using such a workflow library can be useful to implement a business rules visually or programmatically.
+
+![elsa-overview](./elsa-overview.gif)
+
+This article shows how we can use this workflow library within our ABP-based application. We will start with a couple of examples and then we will integrate the **Elsa Dashboard** (you can see it in the above gif) into our application to be able to design our workflows visually.
+
+## Source Code
+
+You can find source code of the sample used in this article [here](https://github.com/abpframework/abp-samples/tree/elsa-demo).
+
+## Create the Project
+
+In this article, I will create a new startup template with EF Core as a database provider and MVC/Razor-Pages for UI framework.
+
+> If you already have a project with MVC/Razor-Pages or Blazor UI, you don't need to create a new startup template, you can directly implement the following steps to your existing project. In other words you can skip this section.
+
+* Before starting to the development, we will create a new solution named `ElsaDemo` (or whatever you want). We will create a new startup template with EF Core as a database provider and MVC/Razor-Pages for UI framework by using the [ABP CLI](https://docs.abp.io/en/abp/latest/CLI):
+
+```bash
+abp new ElsaDemo
+```
+
+* Our project boilerplate will be ready after the download is finished. Then, we can open the solution in the Visual Studio (or any other IDE).
+
+* First of all we need to create database migrations and apply these migrations to our database. So, open your terminal in the `ElsaDemo.EntityFrameworkCore.DbMigrations` project directory and run the following command to create the initial migration.
+
+```bash
+dotnet ef migrations add Initial
+```
+
+* After the initial migration created, we can run the `ElsaDemo.DbMigrator` project to apply migration into our database and seed initial data.
+
+* After database and initial data created, we can run the `ElsaDemo.Web` to see our UI working properly.
+
+> Default admin username is **admin** and password is **1q2w3E***
+
+## Creating First Workflow (Hello World to Workflows)
+
+We can start with creating our first workflow. Let's start with creating basic hello-world workflow.
+
+### Install Packages
+
+We need to install two packages: `Elsa` and `Elsa.Activities.Console` into our `ElsaDemo.Web` project. You can add these two packages with the following command:
+
+```bash
+dotnet add package Elsa
+dotnet add package Elsa.Activities.Console
+```
+
+* After the packages installed, we can define our first workflow. To do this, create a folder named **Workflows** and in this folder create a class named `HelloWorld`.
+
+```csharp
+using Elsa.Activities.Console;
+using Elsa.Builders;
+
+namespace ElsaDemo.Web.Workflows
+{
+    public class HelloWorld : IWorkflow
+    {
+        public void Build(IWorkflowBuilder builder) => builder.WriteLine("Hello World from Elsa!");
+    }
+}
+```
+
+* In here we've basically implemented the `IWorkflow` interface which only has one method named **Build**. In this method, we can define our workflow's executing steps (activities). 
+
+* As you can see in the example above, we've used a activity named **WriteLine**, which writes a line of text to the console. We could also chain the activities one to another.
+
+> "An activity is an atomic building block that represents a single executable step on the workflow." - [Elsa Core Definition](https://elsa-workflows.github.io/elsa-core/docs/next/concepts/concepts-workflows#activity)
+
+* After defining our workflow, we need to register it. To do that, open your `ElsaDemoWebModule` class and update your `ElsaDemoWebModule` with the following lines. Most of the codes are abbreviated for simplicity.
+
+```csharp
+using ElsaDemo.Web.Workflows;
+using Elsa.Services;
+
+public override void ConfigureServices(ServiceConfigurationContext context)
+{
+    var hostingEnvironment = context.Services.GetHostingEnvironment();
+    var configuration = context.Services.GetConfiguration();
+
+    //...
+
+    ConfigureElsa(context); //add this line for Elsa's service registrations
+}
+
+private void ConfigureElsa(ServiceConfigurationContext context)
+{
+    context.Services.AddElsa(options =>
+    {
+        options
+            .AddConsoleActivities()
+            .AddWorkflow<HelloWorld>();
+    });
+}
+
+public override void OnApplicationInitialization(ApplicationInitializationContext context)
+{
+    //...
+
+    var workflowRunner = context.ServiceProvider.GetRequiredService<IBuildsAndStartsWorkflow>();
+    workflowRunner.BuildAndStartWorkflowAsync<HelloWorld>();
+}
+```
+
+* Here we basically, configured Elsa's services in our `ConfigureServices` method and after that in our `OnApplicationInitialization` method we get workflow runner (**IBuildsAndStartsWorkflow**) and started the our `HelloWorld` workflow.
+
+* If we run the application and examine the console outputs, we should see the message that we defined in our workflow.
+
+![hello-world-workflow](./hello-world-workflow.jpg)
+