Update README with metadata transition changes

- Use 'evolution.json' instead of 'proposals.json'
- Add warning that the schema is in a transition period

Author: dempseyatgithub

README.md

@@ -6,6 +6,9 @@ The `swift-evolution-metadata-extractor` tool reads the main branch of the `swif
 
 In addition to the extraction tool, this package vends the `EvolutionMetadataModel` library which provides types for encoding and decoding evolution metadata JSON files. This library is intended for use by clients of the metadata file that drives the evolution dashboard. 
 
+> [!WARNING] 
+> The schema of the metadata generated by this tool is in the midst of a transition, detailed in this thread in the Swift forums: [Swift Evolution Metadata Transition](https://forums.swift.org/t/swift-evolution-metadata-transition/71387). Until the transition period ends in late July 2024, the generated `evolution.json` file and `EvolutionMetadataModel` types will contain properties that will be removed at the end of the transition. Please see the Swift forum thread for full details.
+
 ## Usage
 
 `swift-evolution-metadata-extractor`
@@ -14,7 +17,7 @@ Running the command with no arguments will do the following:
 1. Fetch listing of current proposals from `swift-evolution` repository
 2. Fetch previous metadata results from `swift.org`
 3. Fetch and extract metadata from proposals with changes
-4. Write the extracted metadata to a `proposal.json` file in the same directory as the command
+4. Write the extracted metadata to a `evolution.json` file in the same directory as the command
 
 The default behavior can be configured in various ways.
 
@@ -28,7 +31,7 @@ If the specified path does not exist, the tool will attempt to create the necess
 
   `swift-evolution-metadata-extractor --output-path ~/Desktop`  
 
-  > Will write output to `~/Desktop/proposals.json`
+  > Will write output to `~/Desktop/evolution.json`
 
 - Use a path ending with a file extension to specify path and file name: