Implemented GitHub Actions support

.github/workflows/macos.yml

@@ -0,0 +1,24 @@
+name: MacOS
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  build:
+
+    runs-on: macos-latest
+
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@v2
+
+    - name: Configure
+      run: cmake .
+
+    - name: Build
+      run: make -j4

.github/workflows/ubuntu.yml

@@ -0,0 +1,24 @@
+name: Ubuntu
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@v2
+
+    - name: configure
+      run: cmake .
+
+    - name: build
+      run: make -j4

.github/workflows/windows.yml

@@ -0,0 +1,24 @@
+name: Windows
+
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  build:
+
+    runs-on: windows-latest
+
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@v2
+
+    - name: configure
+      run: cmake .
+
+    - name: build
+      run: cmake --build . -j4

README.md

@@ -1,5 +1,9 @@
 # Etc2Comp - Texture to ETC2 compressor
 
+![Ubuntu](https://github.com/google/etc2comp-1/workflows/Ubuntu/badge.svg)
+![MacOS](https://github.com/google/etc2comp-1/workflows/MacOS/badge.svg)
+![Windows](https://github.com/google/etc2comp-1/workflows/Windows/badge.svg)
+
 Etc2Comp is a command line tool that converts textures (e.g. bitmaps)
 into the [ETC2](https://en.wikipedia.org/wiki/Ericsson_Texture_Compression)
 format. The tool is built with a focus on encoding performance