add profiling guide to contributor docs

CONTRIBUTING.md

@@ -132,6 +132,34 @@ If you want to trigger a debug session from VSCode, you can use the following sn
 }
 ```
 
+## Profiling
+
+If you want to investigate what's contributing to CPU usage you can add the following to the top of the `main()` function in `main.go`
+
+```go
+import "runtime/pprof"
+
+func main() {
+	f, err := os.Create("cpu.prof")
+	if err != nil {
+		log.Fatal("could not create CPU profile: ", err)
+	}
+	defer f.Close()
+	if err := pprof.StartCPUProfile(f); err != nil {
+		log.Fatal("could not start CPU profile: ", err)
+	}
+	defer pprof.StopCPUProfile()
+	...
+```
+
+Then run lazygit, and afterwards, from your terminal, run:
+
+```sh
+go tool pprof --web cpu.prof
+```
+
+That should open an application which allows you to view the breakdown of CPU usage.
+
 ## Testing
 
 Lazygit has two kinds of tests: unit tests and integration tests. Unit tests go in files that end in `_test.go`, and are written in Go. For integration tests, see [here](https://github.com/jesseduffield/lazygit/blob/master/pkg/integration/README.md)