Update README.md (pytorch#959)

* Update README.md

Add more info on how to install Cmake on macOS and how to resolve potential packages issue in Xcode.

* Update README.md

README.md

@@ -367,6 +367,8 @@ The following assumes you've completed the steps for [Setting up ExecuTorch](#se
 #### Requirements
 - [Xcode](https://apps.apple.com/us/app/xcode/id497799835?mt=12/) 15.0 or later
 - [Cmake](https://cmake.org/download/) 3.19 or later
+  - Download and open the macOS `.dmg` installer and move the Cmake app to `/Applications` folder.
+  - Install Cmake command line tools: `sudo /Applications/CMake.app/Contents/bin/cmake-gui --install`
 - A development provisioning profile with the [`increased-memory-limit`](https://developer.apple.com/documentation/bundleresources/entitlements/com_apple_developer_kernel_increased-memory-limit) entitlement.
 
 
@@ -376,6 +378,15 @@ The following assumes you've completed the steps for [Setting up ExecuTorch](#se
     ```bash
     open et-build/src/executorch/examples/demo-apps/apple_ios/LLaMA/LLaMA.xcodeproj
     ```
+
+    > Note: If you're running into any issues related to package dependencies, close Xcode, clean some of the caches and/or the build products, and open the Xcode project again:
+    > ```bash
+    > rm -rf \
+    >   ~/Library/org.swift.swiftpm \
+    >   ~/Library/Caches/org.swift.swiftpm \
+    >   ~/Library/Caches/com.apple.dt.Xcode \
+    >   ~/Library/Developer/Xcode/DerivedData
+    > ```
 2. Click the Play button to launch the app in the Simulator.
 
 3. To run on a device, ensure you have it set up for development and a provisioning profile with the `increased-memory-limit` entitlement. Update the app's bundle identifier to match your provisioning profile with the required capability.