Add notice about security-sensitive experimental code to experimental README. (#9936)

It renders like this:
https://github.com/langchain-ai/langchain/tree/pg/experimental-readme/libs/experimental


![image](https://github.com/langchain-ai/langchain/assets/2348618/a5f9569d-96f6-44c6-8559-921adb3e337d)

libs/experimental/README.md

@@ -1,3 +1,16 @@
 # 🦜️🧪 LangChain Experimental
 
-This repository holds more experimental LangChain code.
+This package holds experimental LangChain code, intended for research and experimental
+uses.
+
+> [!WARNING]
+> Portions of the code in this package may be dangerous if not properly deployed
+> in a sandboxed environment. Please be wary of deploying experimental code
+> to production unless you've taken appropriate precautions and
+> have already discussed it with your security team.
+
+Some of the code here may be marked with security notices. However,
+given the exploratory and experimental nature of the code in this package,
+the lack of a security notice on a piece of code does not mean that
+the code in question does not require additional security considerations
+in order to be safe to use.