Document how to access alpha/beta prerelease versions of Flow using Maven

[basil-bourque]

Document how to access the alpha/beta releases. Ideally, explain how to modify a working Project Base Maven-driven POM file to work with pre-releases.

The Get Started page of the Vaadin Flow site makes no mention of how to access the alpha or beta pre-release versions.

The instructions for Vaadin 8 do not work with a Project Base project.

On Stack Overflow, Change Maven-driven project based on Vaadin 11 to Vaadin 12 alpha version? has gone unanswered.

On the Vaadin Forums, Update a Vaadin 10 project to Vaadin 11 beta? has suggestions that unfortunately do not work.

[basil-bourque]

Perhaps this request is now moot as the starter-packs page began offering a tab for the latest pre-release version.

https://vaadin.com/start/pre-release

Nevertheless, it might be useful to document the steps to take to convert an existing project to use the alpha or beta version.

[ZheSun88]

made a PR to add the needed instructions to release note for prerelease.