Missing reference documentation for the configuration file

[guillaume-perreal]

I have found no reference documentation for the content of phive.xml (nor online schema, by the way).

[vpassapera]

Same.

[glensc]

Neither is there info how to let phive to create it, see the confusion in threads of:

• https://github.com/phar-io/phive/issues/332

[glensc]

Also, docs should updated that there are two files:

Phive stores its state in either .phive/phars.xml (newer versions) or phive.xml (older versions)

I personally prefer phive.xml, not sure what's the reasoning for a hidden folder with a single file.