Add some docs about thread safety

Author: oschwald

README.rst

@@ -94,6 +94,16 @@ The module will return an ``InvalidDatabaseError`` if the database is corrupt
 or otherwise invalid. A ``ValueError`` will be thrown if you look up an
 invalid IP address or an IPv6 address in an IPv4 database.
 
+Thread Safety
+-------------
+
+Both the C extension and pure Python implementations are safe for concurrent
+reads and support Python 3.13+ free-threading. The C extension provides
+free-threading support on platforms with pthread support (such as Linux and
+macOS) and Windows. On other platforms, the extension will use GIL-based
+protection. Calling ``close()`` while reads are in progress may cause
+exceptions in those threads.
+
 Requirements
 ------------
 

maxminddb/reader.py

@@ -288,7 +288,10 @@ def _load_buffer(
         return filename
 
     def close(self) -> None:
-        """Close the MaxMind DB file and returns the resources to the system."""
+        """Close the MaxMind DB file and returns the resources to the system.
+
+        Calling this method while reads are in progress may cause exceptions.
+        """
         with contextlib.suppress(AttributeError):
             self._buffer.close()  # type: ignore[union-attr]