contrib: Add asmap-tool

Co-authored-by: Pieter Wuille <pieter@wuille.net>

contrib/asmap/README.md

@@ -0,0 +1,12 @@
+# ASMap Tool
+
+Tool for performing various operations on textual and binary asmap files,
+particularly encoding/compressing the raw data to the binary format that can
+be used in Bitcoin Core with the `-asmap` option.
+
+Example usage:
+```
+python3 asmap-tool.py encode /path/to/input.file /path/to/output.file
+python3 asmap-tool.py decode /path/to/input.file /path/to/output.file
+python3 asmap-tool.py diff /path/to/first.file /path/to/second.file
+```