Added autoloads and definition of go-mode-hook

The commit is mostly about installation issues and configuration docs:
* added _go-mode-autoloads.el_ module. Standard go distribution ships with go-mode-load.el module, but there is convention for go-mode-autoloads.el (from package.el).
* added _go-mode-hook_ variable definition
* updated REDME for installation instruction and flymake-go notice