docs: update readme

Author: mob-sakai

README.md

@@ -5,11 +5,14 @@ Change the C# compiler (csc) used on your Unity project, as you like!
 
 [![](https://img.shields.io/npm/v/com.coffee.csharp-compiler-settings?label=openupm&registry_uri=https://package.openupm.com)](https://openupm.com/packages/com.coffee.csharp-compiler-settings/)
 [![](https://img.shields.io/github/v/release/mob-sakai/CSharpCompilerSettingsForUnity?include_prereleases)](https://github.com/mob-sakai/CSharpCompilerSettingsForUnity/releases)
-[![](https://img.shields.io/github/release-date/mob-sakai/CSharpCompilerSettingsForUnity.svg)](https://github.com/mob-sakai/CSharpCompilerSettingsForUnity/releases)
-![](https://img.shields.io/badge/unity-2018.3%20or%20later-green.svg)
-[![](https://img.shields.io/github/license/mob-sakai/CSharpCompilerSettingsForUnity.svg)](https://github.com/mob-sakai/CSharpCompilerSettingsForUnity/blob/upm/LICENSE.txt)
-[![PRs Welcome](https://img.shields.io/badge/PRs-welcome-orange.svg)](http://makeapullrequest.com)
-[![](https://img.shields.io/twitter/follow/mob_sakai.svg?label=Follow&style=social)](https://twitter.com/intent/follow?screen_name=mob_sakai)
+[![](https://img.shields.io/github/release-date/mob-sakai/CSharpCompilerSettingsForUnity.svg)](https://github.com/mob-sakai/CSharpCompilerSettingsForUnity/releases)  [![](https://img.shields.io/github/license/mob-sakai/CSharpCompilerSettingsForUnity.svg)](https://github.com/mob-sakai/CSharpCompilerSettingsForUnity/blob/master/LICENSE.txt)
+[![PRs Welcome](https://img.shields.io/badge/PRs-welcome-orange.svg)](http://makeapullrequest.com)  
+![](https://img.shields.io/badge/Unity%202018.3%20or%20later-supported-blue.svg)
+![](https://img.shields.io/badge/Unity%202019.x-supported-blue.svg)
+![](https://img.shields.io/badge/Unity%202020.x-supported-blue.svg)  
+[![Test](https://mob-sakai.testspace.com/spaces/130862/badge?token=43a50d2fc998aa362d36934597de0c84527e5690)](https://mob-sakai.testspace.com/spaces/130862)
+[![CodeCoverage](https://mob-sakai.testspace.com/spaces/130862/metrics/99758/badge)](https://mob-sakai.testspace.com/spaces/130862/current/Code%20Coverage/Code%20Coverage")
+![GitHub Workflow Status](https://img.shields.io/github/workflow/status/mob-sakai/CSharpCompilerSettingsForUnity/unity-test)
 
 << [Description](#description) | [Installation](#installation) | [Usage](#usage) | [Contributing](#contributing) >>
 
@@ -92,6 +95,7 @@ Let's enjoy C# 8.0 features with your Unity project!
 * If `dotnet` is required, install it automatically.
 * `CompilerType.BuiltIn` compiler option to disable this plugin.
 * `Enable Logging` option to display compilation log.
+* `Allow Nullable` option to enable [Nullable reference types (C# 8.0)](https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/nullable-reference-types).
 
 [OpenSesame.Net.Compilers]: https://www.nuget.org/packages/OpenSesame.Net.Compilers
 [OpenSesame.Net.Compilers.Toolset]: https://www.nuget.org/packages/OpenSesame.Net.Compilers.Toolset
@@ -197,7 +201,7 @@ The project setting asset for C# Compiler will be saved in `ProjectSettings/CSha
 ### For C# 8.0 features
 
 [C# 8.0 features](https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-8)  
-[samples](https://github.com/mob-sakai/CSharpCompilerSettingsForUnity/tree/develop/Assets/C%23%208.0%20Features)
+[samples](https://github.com/mob-sakai/CSharpCompilerSettingsForUnity/tree/sandbox/Assets/C%23%208.0%20Features)
 
 If you want to use the C# 8.0 features, set it up as follows:
 
@@ -223,7 +227,7 @@ Some features required external library.
 ### For C# 9.0 features (preview)
 
 [C# 9.0 features](https://docs.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-9)  
-[samples](https://github.com/mob-sakai/CSharpCompilerSettingsForUnity/tree/develop/Assets/C%23%209.0%20Features)
+[samples](https://github.com/mob-sakai/CSharpCompilerSettingsForUnity/tree/sandbox/Assets/C%23%209.0%20Features)
 
 If you want to use the C# 9.0 features, set it up as follows:
 
@@ -248,7 +252,7 @@ Issues are very valuable to this project.
 ### Pull Requests
 
 Pull requests are, a great way to get your ideas into this repository.  
-See [CONTRIBUTING.md](/../../blob/develop/CONTRIBUTING.md) and [develop](https://github.com/mob-sakai/CSharpCompilerSettingsForUnity/tree/develop) branch.
+See [CONTRIBUTING.md](https://github.com/mob-sakai/CSharpCompilerSettingsForUnity/blob/master/CONTRIBUTING.md) and [sandbox](https://github.com/mob-sakai/CSharpCompilerSettingsForUnity/tree/sandbox) branch.
 
 ### Support
 
@@ -267,12 +271,13 @@ With your support, I can spend more time on development. :)
 
 ## Author
 
-[mob-sakai](https://github.com/mob-sakai)
-[![](https://img.shields.io/twitter/follow/mob_sakai.svg?label=Follow&style=social)](https://twitter.com/intent/follow?screen_name=mob_sakai) 
+* [mob-sakai](https://github.com/mob-sakai) [![](https://img.shields.io/twitter/follow/mob_sakai.svg?label=Follow&style=social)](https://twitter.com/intent/follow?screen_name=mob_sakai) ![GitHub followers](https://img.shields.io/github/followers/mob-sakai?style=social)  
+![](https://user-images.githubusercontent.com/12690315/96912074-bcede500-14dc-11eb-8acd-3fdd8d0e4606.png)  
 
 ## See Also
 
 * GitHub page : https://github.com/mob-sakai/CSharpCompilerSettingsForUnity
 * Releases : https://github.com/mob-sakai/CSharpCompilerSettingsForUnity/releases
 * Issue tracker : https://github.com/mob-sakai/CSharpCompilerSettingsForUnity/issues
-* Change log : https://github.com/mob-sakai/CSharpCompilerSettingsForUnity/blob/upm/CHANGELOG.md
+* Change log : https://github.com/mob-sakai/CSharpCompilerSettingsForUnity/blob/master/CHANGELOG.md
+* Test Space : https://mob-sakai.testspace.com/spaces/130862/current