Add quick instructions for building the Zip. (#2819)

Added some instructions to build the zip. Will expand further with releasing information.

Author: ryanwilson

ZipBuilder/README.md

@@ -1,7 +1,74 @@
 # Firebase Zip File Builder
 
 This project builds the Firebase iOS Zip file for distribution.
-More instructions to come.
+
+## Overview
+
+This is a small Swift Package Manager project that allows users to package a Firebase iOS Zip file. With no launch
+arguments, it will use the most recent public versions of all SDKs included in the zip file.
+
+It was designed to fail fast with an explanation of what went wrong, so you can fix issues or dig in without having to dig
+too deep into the code.
+
+## Requirements
+
+In order to build the Zip file, you will need:
+
+- Xcode 10.1
+- CocoaPods
+- An internet connection to fetch CocoaPods
+
+## Running the Tool
+
+You can run the tool with `swift run ZipBuilder [ARGS]` or generate an Xcode project with
+`swift package generate-xcodeproj` and run within Xcode.
+
+In the near future, releases will be built via a builder server instead of on the release engineer's machine, making these
+instructions more of a reference to understand what's going on instead of how to build it yourself.
+
+## Launch Arguments
+
+See `main.swift` and the `LaunchArgs` struct for information on specific launch arguments.
+
+You can pass in launch arguments with Xcode by clicking "ZipBuilder" beside the Run/Stop buttons, clicking "Edit
+Scheme" and adding them in the "Arguments Passed On Launch" section.
+
+### Common Arguments
+
+These arguments assume you're running the command from the `ZipBuilder` directory.
+
+**Required** arguments:
+- `-templateDir $(pwd)/Template`
+  - This should always be the same.
+- `-coreDiagnosticsDir <PATH_TO_FirebaseCoreDiagnostics.framework>`
+  - Needed to overwrite the existing Core Diagnostics framework.
+
+Optional comon arguments:
+- `-updatePodRepo false`
+  - This is for speedups when `pod repo update` has already been run recently.
+
+For release engineers (Googlers packaging an upcoming Firebase release) these commands should also be used:
+-  `-customSpecRepos sso://cpdc-internal/firebase`
+  - This pulls the latest podspecs from the CocoaPods staging area.
+- `-releasingSDKs <PATH_TO_current.textproto>` and
+- `-existingVersions <PATH_TO_all_firebase_ios_sdks.textproto>`
+  - Validates the version numbers fetched from CocoaPods staging against the expected released versions from these
+    textprotos.
+
+Putting them all together, here's a common command to build a releaseable Zip file:
+
+```
+swift run ZipBuilder -templateDir $(pwd)/Template -updatePodRepo false \
+-coreDiagnosticsDir /private/tmp/tmpUqBxKN/FirebaseCoreDiagnostics.framework \
+-releasingSDKs <PATH_TO_current.textproto> \
+-existingVersions <PATH_TO_all_firebase_ios_sdks.textproto> \
+-customSpecRepos sso://cpdc-internal/firebase
+```
+
+## Debugging
+
+You can generate an Xcode project for the tool by running `swift package generate-xcodeproj` in this directory.
+See the above instructions for adding Launch Arguments to the Xcode build.
 
 ## Priorities