Add instructions to install plugins via Homebrew (#1318)

README.md

@@ -130,9 +130,15 @@ the following line to your `Podfile`:
 ```ruby
     pod 'gRPC-Swift-Plugins'
 ```
-
 The plugins are available in the `Pods/gRPC-Swift-Plugins/` folder afterwards.
 
+#### Homebrew
+
+The plugins are available from [homebrew](https://brew.sh) and can be installed with:
+```bash
+    $ brew install swift-protobuf grpc-swift
+```
+
 ## Examples
 
 gRPC Swift has a number of tutorials and examples available. They are split