document {Year} in footer

exampleSite/content/about.md

@@ -44,9 +44,11 @@ The page footer can be defined in `.Params.footer`, and the text is treated as M
 
 ```
 params:
-  footer: "© [Yihui Xie](https://yihui.org) 2017 -- 2021"
+  footer: "© [Yihui Xie](https://yihui.org) 2017 -- {Year}"
 ```
 
+Here `{Year}` means the year in which the site is built (usually the current year).
+
 # Custom layouts
 
 There are two layout files under `layouts/partials/` that you may want to override: `head_custom.html` and `foot_custom.html`. This is how you inject arbitrary HTML code to the head and foot areas. For example, this site has a file `layouts/partials/foot_custom.html` to support LaTeX math via MathJax and center images automatically: