Added github build script

Author: rds1983

.github/FUNDING.yml

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+patreon: rds1983
+custom: https://boosty.to/rds1983

.github/workflows/build-and-publish.yml

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+name: Build & Publish
+
+on:
+  workflow_dispatch:
+
+jobs:
+  BuildAndPublish:
+    runs-on: windows-latest
+
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        submodules: recursive
+    - name: Setup .NET Core
+      uses: actions/setup-dotnet@v1
+      with:
+        dotnet-version: '3.1.x'
+    - name: Build StbImageWriteSharp
+      run: dotnet build src\StbImageWriteSharp.csproj --configuration Release
+    - name: Install NuGet
+      uses: NuGet/setup-nuget@v1
+    - name: Publish StbImageWriteSharp to NuGet
+      run: nuget.exe push src\bin\Release\StbImageWriteSharp.*.nupkg ${{secrets.NUGET_APIKEY}} -Source https://api.nuget.org/v3/index.json