close #107: add a note to reminder users that URLs should not contain…

… spaces unless they are enclosed in `<>`

vignettes/intro.Rmd

@@ -58,7 +58,7 @@ Below is a quick summary:
 -   Headings start with a number of `#`'s, e.g., `## level-two heading`.
 
 -   Inline elements: `**strong**`, `_emphasis_`, `~~strikethrough~~`,
-    `[text](link)`, and `![alt](image/path)`.
+    `[text](link)`, and `![alt](image/path)`.[^1]
 
 -   Inline code is written in a pair of backticks, e.g., `` `code` ``. Code
     blocks can be indented, or fenced by ```` ``` ````.
@@ -72,6 +72,11 @@ Below is a quick summary:
 -   Tables are created with `|` as the column separator (i.e., Pandoc's pipe
     table, which can be generated by `knitr::kable(x, "pipe")`).
 
+[^1]: Please note that for links and images, their URLs [should not contain
+    spaces](https://spec.commonmark.org/current/#link-destination). If they do,
+    the URLs must be enclosed in `<>`, e.g.,
+    `![alt](<some dir/a subdir/foo.png>).`
+
 ### Add-on features
 
 In addition to GFM features, the **markdown** package also supports the
@@ -158,18 +163,18 @@ Insert a footnote here.[^1]
 [^1]: This is the footnote.
 ```
 
-The support is limited for LaTeX output at the moment,[^1] and there are two
+The support is limited for LaTeX output at the moment,[^2] and there are two
 caveats if the document is intended to be converted to LaTeX:
 
-[^1]: If you know C, I'll truly appreciate it if you could help with the LaTeX
+[^2]: If you know C, I'll truly appreciate it if you could help with the LaTeX
     implementation in GFM: <https://github.com/github/cmark-gfm/issues/314>
 
 -   The footnote content must be a single paragraph.
 
--   Only numbers[^2] are supported as identifiers, and other types of
+-   Only numbers[^3] are supported as identifiers, and other types of
     identifiers are not recognized.
 
-[^2]: The specific number doesn't matter, as long as it's a unique footnote
+[^3]: The specific number doesn't matter, as long as it's a unique footnote
     number in the document. For example, the first footnote can be `[^100]` and
     the second can be `[^64]`. Eventually they will appear as `[1]` and `[2]`.
     If you use the RStudio visual editor to edit Markdown documents, the