Freshen up shellcheck's directive's page

gistfile1.txt

PROPOSED:

Shellcheck directives allow you to control how `shellcheck` works, and take the form of comments in files:

    hexToAscii() {
      # shellcheck disable=SC2059
      printf "\x$1"
    }

Supported directives are 

### disable
Prevent shellcheck from processing one or more warnings:

    # shellcheck disable=code[,code...]
    statement_where_warning_should_be_disabled

### source
Tell ShellCheck where to find a sourced file (since 0.4.0):

    # shellcheck source=src/examples/config.sh
    . "$(locate_config)"

### shell
Specify the shell for a script (similar to the shebang, if you for any reason don't want to add one) (since [0.4.5](https://github.com/koalaman/shellcheck/issues/581#issuecomment-249437837)):

    # shellcheck shell=sh
    echo foo &> bar

Directives that replace or are immediately after the shebang apply to the entire script. Otherwise, they are scoped to the structure that follows it (such as all branches of a `case` statement, or an entire function).

*Note: There is a [known bug](../issues/1036) in the current version when directives appear within `then` clauses of `if` blocks that causes Shellcheck to report SC1072 on otherwise valid code. Avoid using directives within `then` clauses - instead place them at the top of the `if` block or another enclosing block. This is fixed on the [online version](https://www.shellcheck.net/) and the next release.*

There is no support for scoping a directive to the first structure of the script. In these cases, use a dummy command `true` or `:` and then add directives, such as

    # This directive applies to the entire script
    # shellcheck disable=2086
    true
    # This directive only applies to this function
    # shellcheck disable=2043
    f() {
      ...
    }

Silencing parser errors is purely cosmetic, and will not prevent ShellCheck from continuing.