docs: add a github actions example to the readme [skip ci]

Author: aminya

README.md

@@ -30,6 +30,53 @@ The package can be used locally or from CI services like GitHub Actions. Stay tu
 - choco
 - brew
 
+# Usage
+
+## Inside GitHub Actions
+
+Here is a complete cross-platform example that tests llvm and gcc. It also uses cmake, ninja, conan, cppcheck, and ccache.
+
+`.github/workflows/ci.yml`:
+
+```yaml
+name: ci
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+      - master
+
+jobs:
+  Test:
+    if: "!contains(github.event.head_commit.message, '[skip ci]')"
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os:
+          - windows-2019
+          - ubuntu-20.04
+          - macos-10.15
+        compiler:
+          - llvm
+          - gcc
+    steps:
+      - name: Setup Cpp
+        uses: aminya/setup-cpp@master
+        with:
+          compiler: ${{ matrix.compiler }}
+          cmake: true
+          ninja: true
+          conan: true
+          cppcheck: true
+          ccache: true
+```
+
+In the `compiler` entry, you can specify the version after a `-`. For example, `llvm-11`.
+
+For the tools, instead of `true`, which choses the default version, you can pass a specific version.
+
 ### Incomplete
 
 - [ ] msvc. It is implemented, but has bugs. See [this issue](https://github.com/aminya/cpp/issues/1)