Updatee workflow actions to target multiple .NET versions

.github/workflows/dotnet.yml

@@ -10,13 +10,16 @@ jobs:
   build:
 
     runs-on: ubuntu-latest
-
+    strategy:
+      matrix:
+        dotnet: [ '2.1.x', '3.1.x', '5.0.x' ]
+    name: .NET ${{ matrix.dotnet }}
     steps:
     - uses: actions/checkout@v2
     - name: Setup .NET
       uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: 5.0.x
+        dotnet-version: ${{ matrix.dotnet }}
     - name: Restore dependencies
       run: dotnet restore
     - name: Build

README.md

@@ -1,5 +1,7 @@
 # SharpNBT
 
+[![.NET](https://github.com/ForeverZer0/SharpNBT/actions/workflows/dotnet.yml/badge.svg)](https://github.com/ForeverZer0/SharpNBT/actions/workflows/dotnet.yml)
+
 A CLS-compliant implementation of the Named Binary Tag (NBT) specification, written in pure C# with no external dependencies and targeting .NET Standard 2.1 to support a wide variety of .NET implementations on all platforms.
 
 ## Features