docs: atInit documentation/new-notes/PR-558.md added.

documentation/new-notes/PR-558.md

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+# New atInit Command Added
+
+This pull request introduces a new IOC shell command `atInit` to EPICS Base, which allows users to schedule arbitrary commands to be executed automatically after the IOC initialization phase (`iocInit`).
+
+The module allows you to write better-structured ioc shell files to include in your startup scripts without tracking where iocInit is located (and how IOC is deployed) e.g.:
+- to achieve the best maintainability,
+- to improve writing boot sequences,
+- to improve IOC startup flexibility and scripting capabilities.
+
+## Features
+
+- Define commands that run after IOC initialization completes.
+- Overwrites ```autosave``` to ensure proper configuration of the required PV.
+- Supports any valid IOC shell command as an argument.
+- Example usages:
+  - `atInit "dbpf <PV> <VAL>"`
+  - `atInit "date"`
+
+## Testing
+
+- Unit tests have been added to verify:
+  - Registration and execution of commands scheduled with `atInit`.
+  - Proper handling of empty or invalid commands without crashing the IOC.
+  - Execution order and environment variable effects.
+
+---
+
+This note documents the functionality introduced in PR #558.