Add Emacs extension to supporting tools

Since early 2025 there is an extension for GNU Emacs under development to make
use of dev containers.  Since its maturity is sufficient for everyday use, it
worth to be added to the page about supporting tools.

Author: johannes-mueller

docs/specs/supporting-tools.md

@@ -38,6 +38,10 @@ Visual Studio added dev container support in Visual Studio 2022 17.4 for C++ pro
 
 You may learn more in the [announcement blog post](https://devblogs.microsoft.com/cppblog/dev-containers-for-c-in-visual-studio/).
 
+### Emacs
+
+[GNU Emacs](https://www.gnu.org/software/emacs/) can make use of dev containers using the community extension [devcontainer.el](https://johannes-mueller.github.io/devcontainer.el/).  The extension is available by the popular package registry [MELPA](https://melpa.org).  It can also be installed directly from the source code that is available on [GitHub](https://github.com/johannes-mueller/devcontainer.el).
+
 ## Tools
 
 ### Dev Container CLI