Add release, bioconda downloads, and citation badges to README

[Copilot]

Adds three shields.io badges to README header for discoverability and metrics visibility.

Badges added:

• GitHub release version (links to releases page)
• Bioconda download count (links to recipe)
• Google Scholar citation search (links to scholar query)

All badges follow existing shields.io format and are placed after current badges (License, Language, Installation) for visual consistency.

[![GitHub release](https://img.shields.io/github/release/tseemann/nullarbor.svg)](https://github.com/tseemann/nullarbor/releases)
[![Bioconda downloads](https://img.shields.io/conda/dn/bioconda/nullarbor.svg)](https://bioconda.github.io/recipes/nullarbor/README.html)
[![Google Scholar](https://img.shields.io/badge/Google%20Scholar-Cite-blue.svg)](https://scholar.google.com/scholar?q=Nullarbor+Seemann)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add badges to README</issue_title> <issue_description>* add verrion / release badge

• add bioconda downloads badge
• add google scholar citation count badge </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes tseemann/nullarbor#302

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