docs: add warning about login method deprecation

Add prominent warning in README that username+password
login method is no longer functional. This makes the
project currently unusable until a fix is implemented.

Author: liblaf

README.md

@@ -1,5 +1,8 @@
 # THU Web Learning Downloader
 
+> [!WARNING]
+> username + password 登陆方式已失效, 致使本项目目前不可用 [#223](https://github.com/liblaf/thu-learn-downloader/issues/223). PR Welcome!
+
 Download everything from Web Learning of Tsinghua University
 
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