Add installation instructions with golangci-lint

Author: nakabonne

README.md

@@ -8,10 +8,16 @@ It helps you find if statements that make your code hard to read, and clarifies
 
 ## Installation
 
+### By go get
+
 ```
 go get github.com/nakabonne/nestif/cmd/nestif
 ```
 
+### By golangci-lint
+
+`nestif` is already integrated with [golangci-lint](https://github.com/golangci/golangci-lint). Please refer to the instructions there and enable it.
+
 ## Usage
 
 ### Quick Start