Add Technical and style guide to the contribution gude

Signed-off-by: tenzen-y <yuki.iwai.tz@gmail.com>

docs/developer-guide.md

@@ -43,6 +43,20 @@ You can undeploy Katib v1beta1 manifests from a Kubernetes cluster as follows:
 make undeploy
 ```
 
+## Technical and style guide
+
+The following guidelines apply primarily to Katib, 
+but other projects like training-operator might also adhere to them. 
+
+When coding:
+
+- Follow [effective go](https://go.dev/doc/effective_go) guidelines.
+
+Testing:
+
+- Use `cmp.Diff` instead of `reflect.Equal`, to provide useful comparisons.
+- Define test cases as maps instead of slices to avoid dependencies on te running order.
+
 ## Modify controller APIs
 
 If you want to modify Katib controller APIs, you have to