Add GitHub action to publish CPM on release (#102)

* add github action to publish cpm on release

* add checkout step

* update readme

.github/workflows/publish.yaml

@@ -0,0 +1,21 @@
+name: Publish
+
+on:
+  push:
+    tags:
+      - '*'
+
+jobs:
+  build:
+    name: Publish CPM.cmake
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v1
+      - name: Upload CPM.cmake to release
+        uses: svenstaro/upload-release-action@v1-release
+        with:
+          repo_token: ${{ secrets.GITHUB_TOKEN }}
+          file: cmake/CPM.cmake
+          asset_name: CPM.cmake
+          tag: ${{ github.ref }}
+          overwrite: true

README.md

@@ -81,7 +81,7 @@ To add CPM to your current project, simply add `cmake/CPM.cmake` to your project
 
 ```bash
 mkdir -p cmake
-wget -O cmake/CPM.cmake https://raw.githubusercontent.com/TheLartians/CPM.cmake/master/cmake/CPM.cmake
+wget -O cmake/CPM.cmake https://github.com/TheLartians/CPM.cmake/releases/latest/download/CPM.cmake
 ```
 
 You can also use CMake to download CPM for you. See the [wiki](https://github.com/TheLartians/CPM/wiki/Adding-CPM) for more details.