Add an AMO download/install link (for Firefox)

Thank you Cimbali and contributors. How good.

Unfortunately, I didn't find/locate this on addons.mozilla.org, but I've found this on GitHub first. Please add a "Download for Firefox" link to README.md, pointing to https://addons.mozilla.org/en-US/firefox/addon/markdown-viewer-webext/. That will save time for people with the same path.

Thank you in advance.

Open README.md and inspect the existing links or installation section. Add a "Download for Firefox" link pointing to the specified addons.mozilla.org URL, then verify that the link is correctly rendered and reachable.

Written by the indexing model from the issue text.