Document webpack change: Allow banner to be placed as a footer

[webpack-bot]

A pull request by @DavidTanner was merged and maintainers requested a documentation change.

See pull request: webpack/webpack#15617

---

What kind of change does this PR introduce?

There is a plugin we are using for version 4 that does the same thing as BannerPlugin, but it could place the text at the bottom of the file instead of the top. This allows the BannerPlugin to place the banner either as a header or footer. No other changes

Did you add tests for your changes?

Yes

Does this PR introduce a breaking change?

No. The new flag defaults to falsy

What needs to be documented once your changes are merged?

The website will need to be updated here to reflect the new optional parameter footer