Add an example for accessing the element index in a for...of loop

[iansan5653]

Add an example using Array.prototype.entries() to access the element index in a for...of loop.

for (const [i, el] of els.entries()) {
  el.name = `Element ${i}`
}

This is preferred over for...in because for...in does not guarantee that elements will be iterated over in the expected order (doesn't matter in the above example but it could in other cases).

And it's more readable than a standard for loop:

for (let i = 0; i < els.length; i++) {
  els[i].name = `Element ${i}`
}

Resolves #216