Update README.md

Author: kfarnung

README.md

@@ -4,8 +4,14 @@ GitHub-compatible Markdown file.
 
 ## Installing
 
+### Firefox
+
 Binaries are available on the [Releases](https://github.com/kfarnung/aoc-to-markdown/releases) page.
 
+### Chrome
+
+The extension must be side-loaded using developer mode.
+
 ## Building
 
  - `npm install`