Explain in docs that v3 is closed off to new features

Refs #18112
[skip sauce]

CONTRIBUTING.md

@@ -116,7 +116,13 @@ When feasible, we aim to report such upstream bugs to the relevant browser vendo
 
 ## Feature requests
 
-Feature requests are welcome. But take a moment to find out whether your idea
+Feature requests are welcome, but please note that they **must target
+[Bootstrap v4](https://github.com/twbs/bootstrap/tree/v4-dev),** because
+Bootstrap v3 is now in maintenance mode and is closed off to new features.
+This is so that we can focus our efforts on Bootstrap v4, the future of the
+framework.
+
+Before opening a feature request, please take a moment to find out whether your idea
 fits with the scope and aims of the project. It's up to *you* to make a strong
 case to convince the project's developers of the merits of this feature. Please
 provide as much detail and context as possible.
@@ -133,6 +139,13 @@ implementing features, refactoring code, porting to a different language),
 otherwise you risk spending a lot of time working on something that the
 project's developers might not want to merge into the project.
 
+In particular, **pull requests that add new features to Bootstrap v3 will be
+rejected.** Bootstrap v3 is now in maintenance mode and is therefore closed
+off to new features, so that we can focus our efforts on Bootstrap v4, the
+future of the framework. Pull requests that add new features should target
+[Bootstrap v4 (the `v4-dev` git branch)](https://github.com/twbs/bootstrap/tree/v4-dev)
+instead, where they will be welcomed and duly considered.
+
 Please adhere to the [coding guidelines](#code-guidelines) used throughout the
 project (indentation, accurate comments, etc.) and any other requirements
 (such as test coverage).