Document include_md

Issue #596

inst/examples/01-introduction.Rmd

@@ -124,6 +124,7 @@ rmd_files:
 ```
 
 Although we have been talking about R Markdown files, the chapter files do not actually have to be R Markdown. They can be plain Markdown files (`.md`), and do not have to contain R code chunks at all. You can certainly use **bookdown** to compose novels or poems!
+However, by default, `.md` files are not included in the automatic collection of files.
 
 At the moment, the major output formats that you may use include `bookdown::pdf_book`, `bookdown::gitbook`, `bookdown::html_book`, and `bookdown::epub_book`. There is a `bookdown::render_book()`\index{bookdown::render\_book()} function similar to `rmarkdown::render()`, but it was designed to render _multiple_ Rmd documents into a book using the output format functions. You may either call this function from command line directly, or click the relevant buttons in the RStudio IDE. Here are some command-line examples:
 

inst/examples/04-customization.Rmd

@@ -205,6 +205,7 @@ We have mentioned `rmd_files` in Section \@ref(usage), and there are more (optio
 -  `history`: similar to `edit`, a link to the edit/commit history of the current page.
 -  `view`: similar to `edit`, a link to source code of the current page. 
 - `rmd_subdir`: whether to search for book source Rmd files in subdirectories (by default, only the root directory is searched). This may be either a boolean (e.g. `true` will search for book source Rmd files in the project directory and all subdirectories) or list of paths if you want to search for book source Rmd files in a subset of subdirectories.
+- `include_md` include md files in search for book source (by default only Rmd files are included).
 - `output_dir`: the output directory of the book (`_book` by default); this setting is read and used by `render_book()`.
 - `clean`: a vector of files and directories to be cleaned by the `clean_book()` function.