README.md - modify the Firefox note a bit

You can add access to local folders for a snap-based application,
you just need to know how. It gets especially confusing if your
working drive happens to be NOT under your $HOME-folder, as
snaps can see ONLY under $HOME. In those cases you MUST
mount that drive to be UNDER $HOME!

README.md

@@ -493,8 +493,7 @@ make build-doc
 This generates the Doxygen documentation under `build-doc/doxygen` folder.
 Run for example: `firefox build-doc/doxygen/index.html &` to view them.
 
-NOTE! If you have a `snap`-based Firefox in use, it will not have access rights to show local files anymore.
-
+NOTE! If you have a `snap`-based Firefox in use, it will not have access rights to show local files by default.
 
 ## Troubleshooting