Updates Custom Field Constructor Documentation

[RommelTJ]

Pull Request Checklist

• [x] Have you read How to write the perfect pull request?
• [x] Have you read through the contributor guidelines?
• [x] Have you referenced any issues you're fixing using commit message keywords?
• [x] Have you added copyright headers to new files?
• [x] Have you checked that both Scala and Java APIs are updated?
• [x] Have you updated the documentation for both Scala and Java sections?
• [x] Have you added tests for any changed functionality?

Helpful things

Fixes

Fixes https://github.com/playframework/playframework/issues/5172

Purpose

Updates the Custom Field Constructor documentation to be more clear. Provides suggested filenames and directories so that a beginner can get started with creating custom field constructors.

If anything is still not clear, or we need further edits, let me know. I'm happy to make them.

Background Context

Outstanding issue with the documentation since October 2015. I created a minimal ("Hello Play") sample project with a minimal custom field constructor to verify the approach.

References

https://github.com/playframework/playframework/issues/5172