Update README.md

Changed the instructions to use and install the extension.

README.md

@@ -1,2 +1,15 @@
 # Pacman Chrome Extension
  Chrome extension popup to play the retro video game Pacman. Saves your highest score, and level. See how far you can get!
+ Note: This has only been tested on windows 10 devices. 
+
+ # To install the extension on the chrome browser:
+
+  1) Open chrome browser (make sure you're logged in) and go to: chrome://extensions/ 
+  2) In the top right-hand corner is developer mode. If developer mode is off, click it to turn it on
+  3) Click the 'load unpacked' button in the upper left side of the page
+  4) Go to the directory of the downloaded extension, and select the folder: Pac-Man-JS. Then click 'Select Folder'
+
+ # To pin the extension to the extensions bar:
+
+  1) Click the little puzzle piece icon in the top right side of the screen, to the right of the search bar.
+  2) Find the Pac-Man Javascript extension icon, then, to the right, click the 'pin' button.