Add alpha build upgrade testing instructions to the CONTRIBUTING.adoc file

A conversation in the Antora Zulip chat (included below) resulted in identifying a gap in the Contributing file.

There are currently no recommended instructions for testing out alpha builds of Antora.

As identified in the conversation, there is a specific way of targeting the alpha version in the package.json file for both the antoar/cli and antora/site-generator-default parameters.

Mark Plummer: Silly question, but what is the best way to upgrade to this alpha.3 release?

I have these two lines under devDependencies in my package.json file: "@antora/cli": "^3.0.0-alpha.1", "@antora/site-generator-default": "^3.0.0-alpha.1",

Would it be a simple as changing the alpha.1 in each line to alpha.3 and then running npm i ?

mojavelinux: When using an alpha, you should type the version exactly without any special symbols.

Mark Plummer: do you mean, use 3.0.0-alpha.1 instead of ^3.0.0-alpha.1 ?

mojavelinux: That's correct.

Mark Plummer: Ok. Thanks. So, when the new alpha comes out, I’d change it to alpha.4 and run ‘npm I’

eskwayrd: Yes. It might also be useful to fully remove the node_modules folder, so that npm i installs exactly the dependencies that are required.

The task is to document these minimum instructions for now and then build upon them after there is a place in the Contributing file.