Bash constants readonly

[Jontified]

In bash guidelines add that constants should be readonly always and that non-constants that are not modified after declaration should be readonly on a best-effort basis. Remove the part of the guideline that says that COMPILER_FLAGS is an example of a constant that can be modified after declaration. After discussion this was concluded not to be the case and that this variable should not be screaming snake case. Update example to reflect this.

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