[ci][docs] added GitHub Actions badge and simplified condition in config (#3179)

* added GitHub Actions badge

* simplified condition in config

.github/workflows/main.yml

@@ -99,12 +99,12 @@ jobs:
           $GITHUB_WORKSPACE/.ci/setup.sh
           $GITHUB_WORKSPACE/.ci/test.sh
       - name: Use conda on Windows
-        if: matrix.os == 'windows-latest' || matrix.os == 'windows-2016' || matrix.os == 'windows-2019'
+        if: startsWith(matrix.os, 'windows')
         uses: goanpeca/setup-miniconda@v1
         with:
           auto-update-conda: false
       - name: Setup and run tests on Windows
-        if: matrix.os == 'windows-latest' || matrix.os == 'windows-2016' || matrix.os == 'windows-2019'
+        if: startsWith(matrix.os, 'windows')
         shell: pwsh -command ". {0}"
         run: |
           $env:BUILD_SOURCESDIRECTORY = $env:GITHUB_WORKSPACE

README.md

 LightGBM, Light Gradient Boosting Machine
 =========================================
 
+[![GitHub Actions Build Status](https://github.com/microsoft/LightGBM/workflows/GitHub%20Actions/badge.svg?branch=master)](https://github.com/microsoft/LightGBM/actions)
 [![Azure Pipelines Build Status](https://lightgbm-ci.visualstudio.com/lightgbm-ci/_apis/build/status/Microsoft.LightGBM?branchName=master)](https://lightgbm-ci.visualstudio.com/lightgbm-ci/_build/latest?definitionId=1)
 [![Appveyor Build Status](https://ci.appveyor.com/api/projects/status/1ys5ot401m0fep6l/branch/master?svg=true)](https://ci.appveyor.com/project/guolinke/lightgbm/branch/master)
 [![Travis Build Status](https://travis-ci.org/microsoft/LightGBM.svg?branch=master)](https://travis-ci.org/microsoft/LightGBM)