Adding documentation around the new disable_init feature

[stefansedich]

Adding some documentation around the disable_init feature as mentioned in #761

[barryib]

Maybe, we can add a snippet for those who need to setup this during CI validation. Something like this :

remote_state.disable_init = tobool("${get_env("TERRAGRUNT_DISABLE_INIT", "false")}")

[stefansedich]

Good idea @barryib just added a snippet showing this.

[brikis98]

Thank you!