Create SwiftPlot_Ubuntu.yml

I've added a new Github Action that builds SwiftPlot on Ubuntu. Please take a look and let me know of any required changes.

@karwa @odmir

Author: KarthikRIyer

.github/workflows/SwiftPlot_Ubuntu.yml

@@ -0,0 +1,28 @@
+name: SwiftPlot Ubuntu
+
+on:
+  push:
+    branches: master
+  pull_request:
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v1
+    - name: Install Freetype
+      run: sudo apt-get install libfreetype6-dev
+    - name: Install Swift Dependencies
+      run: sudo apt-get install clang libicu-dev
+    - name: Download Swift
+      run: wget "https://swift.org/builds/swift-5.1.3-release/ubuntu1804/swift-5.1.3-RELEASE/swift-5.1.3-RELEASE-ubuntu18.04.tar.gz"
+    - name: Install Swift
+      run: |
+        tar xzf swift-5.1.3-RELEASE-ubuntu18.04.tar.gz
+        echo "::add-path::$(pwd)/swift-5.1.3-RELEASE-ubuntu18.04/usr/bin"
+    - name: Build
+      run: swift build -v
+    - name: Run tests
+      run: swift test -v