Update README section about running linters on save
Update README section about running linters on save

The default for `g:ale_lint_on_insert_leave` was recently changed to 1,
so it now needs to be explicitly set to 0 to run linters only when files
are saved.

IngoHeimbach/ale,w0rp/ale,IngoHeimbach/ale,IngoHeimbach/ale,w0rp/ale,w0rp/ale