chore: update README.md

add information about obtaining plugin via Chrome store or utilizing CRX installer to side-load the plugin into Gecko based browsers

Author: mrhoribu

README.md

@@ -3,3 +3,10 @@
 Adds buttons for +/- 10 ranks and +/- 100 ranks to the GSIV trainer page once installed to your browser.
 
 ![simutrainix](https://user-images.githubusercontent.com/1090434/96200054-0069b180-0f27-11eb-8788-9feb8ea19418.png)
+
+## Installation
+### Chromium Browsers (Google Chrome, Microsoft Edge, etc)
+Currently the addon is a manifest v3 plugin that is supported natively via the [Google Chrome Store](https://chromewebstore.google.com/detail/simutrainx/lonampanbgflbjamogdofgbicfemmcgj) for Chromium based browsers.
+
+### Gecko Browsers (Firefox Nightly, Waterfox, Librewolf, Floorp)
+The standalone zip can be used on Firefox Nightly, Librewolf, Waterfox, and other Firefox based forks via loading [CRX Installer](https://addons.mozilla.org/en-US/firefox/addon/crxinstaller/) first. After which, you would load the CRX Installer plugin and browse to the latest .zip release of this addon to get it to work. Due to security restraints, you cannot load this plugin due to it not being signed on normal Mozilla Firefox