Clarify that marking certain member variables const can hinder performance

In the 03-Style document, I read the following sentence:

> If the member variable is not expected to change after the initialization, then mark it const.

However, we know that if, for example, we mark a container as `const`, we prevent it from being moved, possibly hindering performance. Marking fields as `const` is useful to enforce invariants; however, it "interferes" with performance. If both const correctness and performance are important, one could leave the field non-const, make it private and define a field getter.