add documentation regarding thread safety

Author: lonvia

docs/reference/Thread-Safety.md

@@ -0,0 +1,18 @@
+# Thread safety
+
+Object instances of pyosmium are not thread-safe to modify. If you share
+objects like an index, you have to protect write accesses to these objects.
+Concurrent reads are safe.
+
+The library functions themselves are all reentrant and may be used safely from
+different threads.
+
+### Free-threaded Python
+
+Starting with version 4.1, Pyosmium has experimental support for Python
+runtimes with GIL disabled. See the
+[Python Free-Threading Guide](https://py-free-threading.github.io/)
+for more information.
+
+The restrictions mentioned above still apply: write accesses on object need
+to be protected by exclusive locks when using them in multi-threaded context.

mkdocs.yaml

@@ -47,6 +47,7 @@ nav:
       - Indexes: 'reference/Indexes.md'
       - Replication: 'reference/Replication.md'
       - Exceptions: 'reference/Exceptions.md'
+      - Thread Safety: 'reference/Thread-Safety.md'
 
 exclude_docs: |
     .*.swp