[doc] Add install instructions into the readme

README.md

-Dependencies
-------------
+Install it
+----------
 
-  # apt install libzmq3-dev libgtk-3-dev libappindicator3-dev
+Install dependencies:
+
+  # apt install libzmq3-dev libgtk-3-dev libappindicator3-dev golang
+
+Install the systray:
+
+  $ go get 0xacab.org/leap/bitmask-systray
+
+Run bitmask and the systray:
+
+  $ bitmaskd
+  $ ~/go/bin/bitmask-systray