Add changelog entry

Author: vy

src/changelog/.2.x.x/2942_fix_ThreadContext_putAll.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<entry xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+       xmlns="https://logging.apache.org/xml/ns"
+       xsi:schemaLocation="https://logging.apache.org/xml/ns https://logging.apache.org/xml/ns/log4j-changelog-0.xsd"
+       type="fixed">
+  <issue id="2942" link="https://github.com/apache/logging-log4j2/pull/2942"/>
+  <description format="asciidoc">Fix `putAll()` in the default thread context map implementation</description>
+</entry>