d04d4cf README.md: correct tools installation command name
03274ad src/goStatus: present error icon if gopls couldn't be found
465fe7e src/goLanguageServer.ts: prompt to update instead of silent update
607cebe src/welcome: add note about removing go.useLanguageServer setting

Change-Id: I43ff57a9698097b9e05e8e705082912cd41ac3d6