For more Markdown conventions, see RStudio’s R Markdown Reference Guide (link also available through “Help” in RStudio). There are several great books on knitr and RMarkdown now;
R Markdown: The Definitive Guide is particularly
good, and includes the creater of knitr and rmarkdown as a coauthor.
For more on creating dashboards using RMarkdown, see flexdashboard package’s website, which includes many examples
of code and output for these dashboards.
You can post the output of an RMarkdown file for free (note that it will be public) through RPubs. This is also a great site for exploring reports others have created using RMarkdown. For more on posting your own files to RPubs, see here.
A collection of books published with R using RMarkdown is available through
bookdown.org. You can also use RMarkdown to create your own website
with a blog using the blogdown package, which is explained in
blogdown: Creating Websites with R Markdown.