update README with installation instructions

Author: jjw24

README.md

@@ -1,6 +1,9 @@
 # Flow.Launcher.Plugin.BrowserTabs
 Search, open, and close browser tabs. A centralized plugin for managing all open tabs in your browser
 
+### Installation
+Type `pm install Browser Tabs` from flow
+
 ### Usage
 - Activate tab via `Enter` / context menu `Shift + Enter` & select 'Switch to tab'
 - Close tab via `Ctrl + Enter` / context menu `Shift + Enter` & select 'Close tab'