Add link to the janusgraph-visualizer

[porunov]

---

Thank you for contributing to JanusGraph!

In order to streamline the review of the contribution we ask you
to ensure the following steps have been taken:

For all changes:

• Is there an issue associated with this PR? Is it referenced in the commit message?
• Does your PR body contain #xyz where xyz is the issue number you are trying to resolve?
• Has your PR been rebased against the latest commit within the target branch (typically master)?
• Is your initial contribution a single, squashed commit?

For code changes:

• Have you written and/or updated unit tests to verify your changes?
• If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• If applicable, have you updated the LICENSE.txt file, including the main LICENSE.txt file in the root of this repository?
• If applicable, have you updated the NOTICE.txt file, including the main NOTICE.txt file found in the root of this repository?

For documentation related changes:

• Have you ensured that format looks appropriate for the output in which it is rendered?

[li-boxuan]

I notice the below section still mentions [Gremlin-Visualizer](https://github.com/prabushitha/gremlin-visualizer). Shall we somehow remove that and mention this is a fork of that? Or any other wording to avoid confusion.

[porunov]

I just added a sentence to clarify that this was the original repository on which JanusGraph was built on.
I think, it makes sense to leave it in case gremlin-visualizer will decide to diverge in the future. In that case, users will be able to choose between visualizers they prefer more.
That said, I doubt it will diverge, so I'm OK just removing it as well.

[li-boxuan]

This looks good to me!