win build: add instructions to investigate reproducibility failure

contrib/build-wine/README.md

@@ -1,5 +1,4 @@
-Windows binaries
-================
+# Windows binaries
 
 ✓ _These binaries should be reproducible, meaning you should be able to generate
    binaries that match the official releases._
@@ -29,8 +28,7 @@ similar system.
 
 
 
-Code Signing
-============
+## Code Signing
 
 Electrum Windows builds are signed with a Microsoft Authenticode™ code signing
 certificate in addition to the GPG-based signatures.
@@ -57,8 +55,7 @@ certificate/key) and one or multiple trusted verifiers:
 
 
 
-Verify Integrity of signed binary
-=================================
+## Verify Integrity of signed binary
 
 Every user can verify that the official binary was created from the source code in this 
 repository. To do so, the Authenticode signature needs to be stripped since the signature
@@ -73,3 +70,20 @@ This procedure removes the differences between the signed and unsigned binary:
    of 8.
 
 The script `unsign.sh` performs these steps.
+
+## FAQ
+
+### How to investigate diff between binaries if reproducibility fails?
+`pyi-archive_viewer` is needed, for that run `$ pip install pyinstaller`.
+As a first pass overview, run:
+```
+pyi-archive_viewer -l electrum-*.exe1 > f1
+pyi-archive_viewer -l electrum-*.exe2 > f2
+diff f1 f2 > d
+cat d
+```
+Then investigate manually:
+```
+$ pyi-archive_viewer electrum-*.exe1
+? help
+```