Add pack diff command

README.md

@@ -20,6 +20,7 @@ with.
 
 - `m2pack keygen`
 - `m2pack build`
+- `m2pack diff`
 - `m2pack list`
 - `m2pack verify`
 - `m2pack extract`
@@ -57,6 +58,7 @@ Exposed tools:
 
 - `pack_keygen`
 - `pack_build`
+- `pack_diff`
 - `pack_list`
 - `pack_verify`
 - `pack_extract`
@@ -148,6 +150,15 @@ Export a client config header for `m2dev-client-src/src/PackLib/M2PackKeys.h`:
   --output /path/to/m2dev-client-src/src/PackLib/M2PackKeys.h
 ```
 
+Diff a source tree against an archive:
+
+```bash
+./build/m2pack diff \
+  --left /path/to/client/root \
+  --right out/client.m2p \
+  --json
+```
+
 ## Format summary
 
 - Single archive file with a fixed header