Add autoload copying to installation instructions

If you don't copy this file you might get an error such as:

Error detected while processing function <SNR>7_UseConfigFiles..<SNR>7_UseConfig
Files_ExternalCommand..<SNR>7_SpawnExternalParser..<SNR>7_ApplyConfig:
line   96:
E117: Unknown function: editorconfig#ApplyHooks

(see this issue https://github.com/editorconfig/editorconfig/issues/66)