Add known bugs list.

Signed-off-by: Yuhang Zhao <2546789017@qq.com>

BUGS.md

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+# Known Bugs
+
+## Repaint not in time / 重绘不及时
+
+The white background of the window can be seen if it's being resized using the right-bottom corner.
+
+Most frameless applications have this issue. The root cause and it's solution is unknown.
+
+用右下角调整窗口大小时，窗口的白底（白色背景）能被看到。
+
+大多数自定义边框的程序（VS、VSCode、Adobe全家桶、Office全家桶、火狐、Chrome等）都存在这个问题（不过它们被看到的不一定是白色的背景，但能看到窗口自身的背景是肯定的），暂时不知道原因，解决方案也未知。
+
+## Window flickers / 窗口抖动或闪烁
+
+The window flickers when it's being resized using the top or left edge.
+
+Most frameless applications have this issue. The root cause and it's solution is unknown.
+
+用顶边或左边调整窗口大小时，窗口剧烈抖动或闪烁。
+
+大多数自定义边框的程序都存在这个问题，暂时不知道原因，解决方案也未知。
+
+## Unusual memory usage / 内存用量异常
+
+The application uses much more memory after using this repo.
+
+Turn off layered window will solve this. But the window may flicker (see the previous bug).
+
+使用这个项目后，程序的内存用量突然明显增加。
+
+关闭分层窗口可以解决这个问题，但这可能会导致窗口出现抖动或闪烁的问题（见上一个bug）。