Add readme about ios_pod directory and its swift_headers subdirectory

Author: jonsimantov

ios_pod/readme.txt

@@ -0,0 +1,13 @@
+firebase-cpp-sdk/ios_pod directory
+
+This directory contains a Podfile that allows us to install the Firebase iOS SDK
+via CocoaPods. The Podfile here must include all Cocoapods used by the Firebase
+C++ SDK. The pods are extracted as part of the build process, and the header
+files within them are referenced by the Objective-C++ code in this SDK (see
+setup_pod_headers in CMakeLists.txt for details).
+
+The "swift_headers" subdirectory contains copies of the most recent Swift
+bridging headers for the Firebase iOS libraries that are written in Swift,
+enabling them to be called from this SDK's Objective-C++ code. These headers are
+copied from the most recent firebase-ios-sdk release via this repository's
+update-dependencies.yml GitHub Actions workflow.