Add tutorial in README (#4)

Author: stil

.gitignore

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 /.idea
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+.DS_Store

README.md

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-# efmig
-Missing GUI for Entity Framework Core migrations.
+# efmig
+
+This is a multi-platform GUI application that greatly speeds up development when using Entity Framework Core.
+
+![alt text](images/main.png)
+
+
+Why you should use it:
+* You'll no longer need to remember `dotnet ef` commands syntax for creating, removing and generating migration scripts.
+* Dependency on `Microsoft.EntityFrameworkCore.Design` can be dropped inside your project.
+* You can remove `IDesignTimeDbContextFactory<TDbContext>` implementations from your project.
+
+**Important**: This tool **does not** apply migrations on your database. It's designed to be non-destructive and you're expected to generate migration script, review it and run it manually in database client of your choice.
+
+## Configuration
+
+1. Download binaries for your platform from Releases page and launch the `efmig` executable.
+2. Click `[+]` button to create a new configuration profile.
+3. Fill configuration values
+   - Profile name: this is a label of your choice for configuration profile.
+   - dotnet-ef version: See [NuGet page](https://www.nuget.org/packages/dotnet-ef/#versions-body-tab) for available versions. Enter version matching your project.
+   - Microsoft.EntityFrameworkCore.Design version: See [NuGet page](https://www.nuget.org/packages/Microsoft.EntityFrameworkCore.Design/#versions-body-tab) for available versions.
+   - Target framework version: See [this reference table](https://learn.microsoft.com/en-us/dotnet/standard/frameworks#supported-target-frameworks) for available versions.
+   - DbContext .csproj path: pick the path of C# project that holds your DbContext class.
+   - DbContext fully qualified name: this is a full class name of DbContext, for example `MyApp.MyDbContext`.
+   - DbContext configuration code: here you need to configure connection string for `dotnet-ef` to work. See [explanation below](#dbcontext-configuration-code). 
+4. Save profile.
+
+### DbContext configuration code
+
+`optionsBuilder` variable of type `DbContextBuilder<TYourDbContext>` is defined and you are expected to configure connection string using C# syntax.
+
+Postgres example
+```csharp
+var connectionString = "Host=myserver;Username=mylogin;Password=mypass;Database=mydatabase";
+optionsBuilder.UseNpgsql(connectionString);
+```
+
+MySQL example
+```csharp
+var connectionString = "server=localhost;user=myuser;password=mypassword;database=mydb;";
+var serverVersion = ServerVersion.AutoDetect(connectionString);
+optionsBuilder.UseMySql(connectionString, serverVersion);
+```
+
+## Usage
+You can invoke one of the following actions:
+- Create a new migration. Specify its name beforehand and click `[+]` button on the right.
+- List migrations. This action does nothing but lists already existing migrations.
+- Remove most recent migration. This action removes most recent migration from **the code**. Nothing will happen on the actual database.
+- Generate migration script. This action opens default text editor with SQL code of all migrations for you to review.
+
+## Other screenshots
+
+#### Profile configuration
+![alt text](images/profile-settings.png)
+
+#### Migration script output
+![alt text](images/migration-script.png)