Add article about mutating data & public properties

Nobody has claimed this yet.

Per a team discussion, we could add an article that discusses the challenges of passing around mutable objects & arrays to components.

For example, if your component has a public property that takes an array, your component could mutate the array, and the user would not know.

An alternative pattern would be to have separate input/output properties, for example, where the component makes an internal copy from the input property. Mutates the internal copy, and when the user approves changes, copies the internal version to the output property and fires an event.

@justinfagnani also noted the need to clone the copy to the appropriate depth, and maybe override `hasChanged` to check changes from the outside against the internal copy.

Start with the examples and alternative pattern described in the issue; no file or test is named. Done means publishing an article that explains mutation risks for arrays and public properties, input/output patterns, cloning to the appropriate depth, and possibly using hasChanged for external comparisons.

Written by the indexing model from the issue text.