doc: add link to bookup in README

README.md

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 ![CRAN Downloads](https://cranlogs.r-pkg.org/badges/last-month/rmdformats)
 [![R build status](https://github.com/juba/rmdformats/workflows/R-CMD-check/badge.svg)](https://github.com/juba/rmdformats/actions?query=workflow%3AR-CMD-check)
 
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+**Quarto users : ** these templates are only suitable for RMarkdown documents. I published one quarto custom format called [bookup](https://github.com/juba/bookup-html) which is a porting of the `robobook` template.
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 This R package provides ready-to-use HTML output formats and templates for
 RMarkdown documents. The goal is to produce clean documents "out of the box",
 with or without the RStudio IDE.