add tip for the JSON output (#266)

This PR provides a tip when output as JSON due to the current default
`pretty-print` flag

## Current Issue
 
For the orginal json flag provided by
[checker.go](https://github.com/golang/tools/blob/0e7ccc0478c322a6b32fb9f74ebfc42e4f965ee9/go/analysis/internal/checker/checker.go#L510),
it's impossible to get `analysisflags.JSON` from nilaway. There is a
conflict bettwen the `pretty-print` and `json` output currently due to
the code
https://github.com/uber-go/nilaway/blob/e90288479601315af13b7fdd3ccd6b50c53a8e7c/nilaway.go#L43.
As the default `conf.PrettyPrint ` is true, so the output will be messy
when try to output as json.
<img width="1303" alt="image"
src="https://github.com/uber-go/nilaway/assets/12164075/4b199a3b-7151-43a8-ba88-12052859ad00">

Co-authored-by: Yuxin Wang <yuxinw@uber.com>

README.md

@@ -62,6 +62,13 @@ Then, run the linter by:
 nilaway -include-pkgs="<YOUR_PKG_PREFIX>,<YOUR_PKG_PREFIX_2>" ./...
 ```
 
+> [!TIP]  
+> Disable the `pretty-print` flag when output as JSON:
+> ```shell
+> nilaway -json -pretty-print=false -include-pkgs="<YOUR_PKG_PREFIX>,<YOUR_PKG_PREFIX_2>" ./...
+> ```
+
+
 ### golangci-lint (>= v1.57.0)
 
 NilAway, in its current form, can report false positives. This unfortunately hinders its immediate