Add github action -- cross-platform builds on push

Author: medengineer

.github/workflows/linux.yml

@@ -0,0 +1,21 @@
+name: Linux
+
+on: [push]
+
+jobs:
+  build-ubuntu:
+
+    runs-on: ubuntu-latest
+    
+    steps:
+    - uses: actions/checkout@v1
+    - name: configure
+      run: |
+        cd Build && cmake -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Release ..
+        cd ../Resources/Scripts
+        sudo ./install_linux_dependencies.sh
+        sudo cp 40-open-ephys.rules /etc/udev/rules.d
+    - name: build
+      run: cmake --build Build
+#    - name: test
+#      run: cd build && ctest

.github/workflows/osx.yml

@@ -0,0 +1,24 @@
+name: macOS
+
+on: [push]
+
+jobs:
+  build-osx:
+
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [macos-latest]
+    
+    steps:
+    - uses: actions/checkout@v1
+    - name: configure
+      run: |
+        cd Build 
+        cmake -G "Xcode" ..
+    - name: build
+      run: |
+        cd Build 
+        xcodebuild -project open-ephys-GUI.xcodeproj -scheme=ALL_BUILD 
+#    - name: test
+#      run: cd build && ctest

.github/workflows/windows.yml

@@ -0,0 +1,24 @@
+name: Windows
+
+on: [push]
+
+jobs:
+
+  build-windows:
+
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [windows-latest]
+    
+    steps:
+    - uses: actions/checkout@v1
+    - name: configure
+      run: |
+        cd Build 
+        cmake -G "Visual Studio 16 2019" -A x64 ..
+    - name: build
+      run: cmake --build Build
+#    - name: test
+#      run: cd build && ctest
+

README.md

@@ -14,7 +14,9 @@ Our primary user base is scientists performing electrophysiology experiments wit
 
 [![docs](https://img.shields.io/badge/docs-confluence-blue.svg)](https://open-ephys.atlassian.net/wiki/spaces/OEW/pages/491527/Open+Ephys+GUI)
 [![latest release](https://img.shields.io/github/release/open-ephys/plugin-gui.svg)](https://github.com/open-ephys/plugin-GUI/releases)
-![platforms](https://img.shields.io/badge/platforms-macOS%20|%20windows%20|%20linux-lightgray.svg)
+![Linux](https://github.com/medengineer/plugin-GUI/workflows/Linux/badge.svg)
+![OSX](https://github.com/medengineer/plugin-GUI/workflows/macOS/badge.svg)
+![Windows](https://github.com/medengineer/plugin-GUI/workflows/Windows/badge.svg)
 ![language](https://img.shields.io/badge/language-c++-blue.svg)
 [![license](https://img.shields.io/badge/license-GPL3-blue.svg)](https://github.com/open-ephys/plugin-GUI/blob/master/Licenses/Open-Ephys-GPL-3.txt)