Add initial readme content.

README.md

@@ -1,2 +1,87 @@
 # MapTo
-An object to object mapping generator using using Roslyn code generator.
+An object to object mapping generator using using [Roslyn source generator](https://github.com/dotnet/roslyn/blob/master/docs/features/source-generators.md).
+
+## Installation
+```
+dotnet add package MapTo
+```
+
+## Usage
+MapTo creates mappings during compile-time. To indicate which objects it needs to generate the mappings for, simply declare the class as `partial` and annotate it with `MapFrom` attribute.
+
+```c#
+[MapFrom(sourceType: typeof(App.Data.Models.User))]
+public partial class UserViewModel 
+{
+    public string FirstName { get; }
+
+    public string LastName { get; }
+}
+```
+
+If `Data.Models.User` class is defined as follow:
+
+```c#
+namespace App.Data.Models
+{
+    public class User
+    {
+        public User(int id) => Id = id;
+        
+        public int Id { get; }
+
+        public string FirstName { get; set; }
+
+        public string LastName { get; set; }
+
+        public string FullName => $"{FirstName} {LastName}";
+    }
+}
+```
+
+It will generate the following code:
+
+```c#
+// <auto-generated />
+using System;
+
+namespace App.ViewModels
+{
+    public partial class UserViewModel
+    {
+        public UserViewModel(App.Data.Models.User user)
+        {
+            if (user == null) throw new ArgumentNullException(nameof(user));
+            FirstName = user.FirstName;
+            LastName = user.LastName;
+        }
+
+        public static UserViewModel From(App.Data.Models.User user)
+        {
+            return user == null ? null : new UserViewModel(user);
+        }
+    }
+
+    public static partial class UserExtensions
+    {
+        public static UserViewModel ToUserViewModel(this App.Data.Models.User user)
+        {
+            return user == null ? null : new UserViewModel(user);
+        }
+    }
+}
+```
+
+Which makes it possible to get an instance of `UserViewModel` from one of the following ways:
+
+```c#
+var user = new User(id: 10) { FirstName = "John", LastName = "Doe" };
+
+var vm = user.ToUserViewModel();
+
+// OR
+vm = new UserViewModel(user);
+
+// OR
+vm = UserViewModel.From(user);
+```