Simpleini: Add information about portability
This plugin reads and writes files written in a basic line-oriented ini-like format.
It is very simplistic without sections, the [ini](../ini/) plugin and for specifications
-the [ni](../ni/) plugin should be preferred in most cases.
+the [ni](../ni/) plugin should be preferred in most cases. Since the `simpleini` plugin requires
+the GNU C library it **will not work** on operating systems that use another C library
+such as macOS.
## Usage
