docs: add fx plugin documentation to plugins.md

docs/plugins.md

@@ -48,6 +48,14 @@ application without IPC and without forking Kubo.
 Note: We eventually plan to make Kubo usable as a library. However, this
 plugin type is likely the best interim solution.
 
+### fx
+
+Fx plugins let you customize the [fx](https://pkg.go.dev/go.uber.org/fx) dependency graph and configuration,
+by customizing the`fx.Option`s that are passed to `fx` when the IPFS node is initialized.
+
+For example, you can inject custom implementations of interfaces such as [exchange.Interface](https://github.com/ipfs/go-ipfs-exchange-interface)
+or [pin.Pinner](https://github.com/ipfs/go-ipfs-pinner) by adding an option like `fx.Replace(fx.Annotate(customExchange, fx.As(new(exchange.Interface))))`.
+
 ### Internal
 
 (never stable)