Add Github workflow and Readme.

.github/workflows/README.md

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+## Running build.yml locally
+
+### Steps:
+
+1. Install [nektos/act](https://github.com/nektos/act) (Simplest way is to install using homebrew by: `brew install act`)
+2. Install the `gh` command line tool (`brew install gh`) and authenticate it using `gh auth login`.
+3. Make sure your github account has read access to the [cli-binary](https://github.com/PreternaturalAI/cli-binary) repo.
+4. From the root directory `merge`, run the following command:
+
+```
+sudo act -P macos-latest=-self-hosted -W '.github/workflows/build.yml' -s GITHUB_TOKEN="$(gh auth token)"
+```
+
+**Note:** `sudo` is required here because an internal dependency ([`maxim-lobanov/setup-xcode@v1`](https://github.com/maxim-lobanov/setup-xcode/blob/7f352e61cbe8130c957c3bc898c4fb025784ea1e/src/xcode-selector.ts#L51)) uses the `sudo` command and causes `act` execution to wait for a password interactively.

.github/workflows/build.yml

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+name: Build Merge For All Platforms (Xcode 16 & Xcode 15.4)
+on:
+  workflow_dispatch:
+jobs:
+  build:
+    runs-on: macos-latest
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@v3
+      with:
+        path: merge
+    - name: Cache derived data
+      if: ${{ !env.ACT }} # Skipping when run locally.
+      uses: actions/cache@v3
+      with:
+        path: ~/Library/Developer/Xcode/DerivedData
+        key: ${{ runner.os }}-derived-data
+        restore-keys: |
+          ${{ runner.os }}-derived-data
+    - name: Run Preternatural Build (Xcode 16)
+      uses: PreternaturalAI/github-action@main
+      with:
+        command: build
+        derived_data_path: ~/Library/Developer/Xcode/DerivedData
+        xcode-version: '16'
+        build_all_platforms: 'true'
+        working-directory: 'merge'
+    - name: Run Preternatural Build (Xcode 15.4)
+      uses: PreternaturalAI/github-action@main
+      with:
+        command: build
+        derived_data_path: ~/Library/Developer/Xcode/DerivedData
+        xcode-version: '15.4'
+        build_all_platforms: 'true'
+        working-directory: 'merge'