For new checks and feature suggestions

• https://www.shellcheck.net/ (i.e. the latest commit) currently gives no useful warnings about this
• I searched through https://github.com/koalaman/shellcheck/issues and didn't find anything related

While reading through the bash manual I encountered this:

The command substitution $(cat file) can be replaced by the equivalent but faster $(< file). 

It's a relatively minor thing but it might be worth suggesting to users.
Thoughts?

Here's a snippet or screenshot that shows the problem:

#!/bin/bash
var=$(cat file.txt)

Here's what shellcheck currently says:

n/a

Here's what I wanted or expected to see:

Suggestion to use var=$(< file.txt) instead