Mention in README that 32bit systems are not supported.

See #401

Author: joto

README.md

@@ -4,7 +4,8 @@ https://osmcode.org/libosmium
 
 A fast and flexible C++ library for working with OpenStreetMap data.
 
-Libosmium works on Linux, macOS and Windows.
+Libosmium works on Linux, macOS and Windows. You need a 64bit system, 32bit
+systems are not supported.
 
 [![Build Status](https://github.com/osmcode/libosmium/actions/workflows/ci.yml/badge.svg)](https://github.com/osmcode/libosmium/actions)
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