Suggestion: Tell if a variable can be declared as 'readonly'

[nihilus]

For new checks and feature suggestions

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

Many times a novice makes the mistake of not using the 'readonly' keyword for variables intended to be read-only. If a variable is not changed one could get a suggestion to declare it readonly.

[mcandre]

Linters for other programming languages offer this feature. This helps the code to better reflect the author's intention and reduce mistakes.