Update README.md

Some build instructions updates
Added download of custom content
Merge branch 'master' of github.com:CodeCharmLtd/UTLauncher
Add support for MacOSX build and packaging
Fix servers jumping on the list.
Fix to use ping averages, not pings themselves
Updated launcher to new build
Fixed Linux/Mac launching ...
Require newer cmake
Fix memory leak.
Fix another leak that also caused CPU usage to spike when running long time
Run from UE4 executable on Linux and Mac as well
Updates for new build
Updated icon and splash
Fixed windows icon
now based on original master
now based on original master
Fixing conflict!