Disclaimer: This is not legal advice, I am not a lawyer. I have worked in the open source legal space for 15+ years and worked closely with Red Hat Legal in my previous job.
@erickg, what you see in the README.md is what I would expect to see when you fork another project. They are attributing the copyright holder on the original work, and documenting the license that the work is under.
As you add/modify code to your fork, their copyright statement (and license for their copyrighted changes) still apply, so you need to be sure to retain that attribution (in README.md and wherever it appears in the code files, probably in the comment header). What you can do is append your own copyright statement, like this:
Copyright 2021 Elasticsearch B.V.
Copyright 2021 ErickG
Licensed under the Apache License, Version 2.0
You can definitely do this in README.md without issue, and you can make this change to any source files you modify. If you create entirely new files (that do not copy content from existing files), you do not need to include the Elasticsearch copyright attribution statement.
I am assuming, for simplicity, that your fork intends to keep the Apache License, Version 2.0, that you inherited from the upstream fork. It is possible for your changes to be under a different license, but it complicates things (including my answer) quite a bit, so my advice to you would be to keep your fork Apache 2.0.
If you have additional Copyright or License questions, please feel free to ask me, and I will do my best to help you.