Revert "docs: remove obsolete notice"

This reverts commit 44431c2.

Author: ViRb3

README.md

@@ -4,14 +4,24 @@ This is a free and simple builder server for [ios-signer-service](https://github
 
 You only need to configure one builder. If you already configured a CI provider as your builder, you don't need to do anything here. This project is aimed at people who want to have a self-hosted builder.
 
-## Security
+## Important
+
+### Security
 
 This server requires the use of an authentication key so that only the web service can control your builder. However, there is no built-in support for HTTPS or any other form of encryption. Therefore:
 
 > :warning: **Anybody with access to the builder's network can potentially manipulate the builder to execute any code that they want on your machine.**
 
 To prevent this, only deploy this server in a trusted environment, or even better, wrap the server in HTTPS yourself using a reverse proxy like nginx.
 
+### Side effects on your Mac
+
+While the server will do its best to keep changes to your Mac at a minimum, certain changes are inevitable. The biggest one to know is:
+
+> :warning: **Your keychain will be swapped out during signing and restored back afterwards.**
+
+It is highly recommended that you dedicate this Mac exclusively as a builder. Using it for other purposes, especially at the same time as a sign job is running, could lead to random issues.
+
 ## Setup
 
 All the steps should be performed on your builder Mac.