Add protoc-gen-multi to tools list (#13)

Adds it to the collection of tools.

README.md

@@ -5,6 +5,7 @@ A collection of tools written at Buf.
 
 - [license-header](cmd/license-header): A tool to add and update license headers in source code. Also available [for npm](npm-packages/license-header).
 - [protoc-gen-by-dir](cmd/protoc-gen-by-dir): A helper `protoc` plugin to assist in the authoring of custom remote plugins for the Buf Schema Registry to invoke a plugin remotely on a per-directory basis within a single container.
+- [protoc-gen-multi](cmd/protoc-gen-multi): A helper `protoc` plugin to combine multiple protoc plugin invocations into a single plugin.
 
 ## Status: Alpha