pdf: add content may outdated warning

pdf/en-us/meta/template.tex

@@ -166,12 +166,17 @@ $endif$
   \vspace{1ex}
   Last update: \today
 
+  \vspace{1ex}
+  \textbf{Notice}
+
+  \noindent The content in this PDF file may outdated, please check \href{https://changkun.de/modern-cpp}{our website} or \href{https://github.com/changkun/modern-cpp-tutorial}{GitHub repository} for the latest book updates.
+
   \vspace{1em}
   \textbf{\large License}
 
   \noindent This work was written by \href{https://github.com/changkun}{Ou Changkun} and licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. \texttt{\small http://creativecommons.org/licenses/by-nc-nd/4.0/}
 
-  \vspace{8em}
+  \vspace{7em}
   \includegraphics{../../assets/cover-2nd-en}
 
 \end{center}

pdf/zh-cn/meta/template.tex

@@ -172,12 +172,17 @@ $endif$
   \vspace{1ex}
   最后更新 \today - \revision
 
+  \vspace{1ex}
+  \textbf{注意}
+
+  \noindent 此 PDF 的内容可能过期，请检查\href{https://changkun.de/modern-cpp}{本书网站}以及 \href{https://github.com/changkun/modern-cpp-tutorial}{GitHub 仓库}以获取最新内容。
+
   \vspace{1em}
   \textbf{\large 版权声明}
 
   \noindent 本书系\href{https://github.com/changkun}{欧长坤}著，采用“知识共享署名-非商业性使用-禁止演绎 4.0 国际许可协议 (cc by-nc-sa)”进行许可。\texttt{\small http://creativecommons.org/licenses/by-nc-nd/4.0/}
 
-  \vspace{8em}
+  \vspace{7em}
   \includegraphics{../../assets/cover-2nd}
 
 \end{center}