docs: update readme with links

Author: husa

README.md

@@ -1 +1,8 @@
-<a href="https://chrome.google.com/webstore/detail/e-clock/ojhhcfhabhligodffabdhcaoicecaboo"><img src="https://developer.chrome.com/webstore/images/ChromeWebStore_Badge_v2_340x96.png" alt="e-clock" aria-label="e-clock" /></a>
+<p align="center">
+  <a href="https://chrome.google.com/webstore/detail/e-clock/ojhhcfhabhligodffabdhcaoicecaboo">
+    <img src="https://developer.chrome.com/static/docs/webstore/branding/image/HRs9MPufa1J1h5glNhut.png" alt="e-clock" aria-label="e-clock" />
+  </a>
+  <a href="https://addons.mozilla.org/firefox/addon/eclock/">
+    <img src="https://blog.mozilla.org/addons/files/2020/04/get-the-addon-fx-apr-2020.svg" alt="e-clock" aria-label="e-clock" width="496px"/>
+  </a>  
+</p>