Add Documentation of the AppVeyor build process for NVDA forks (#16293)
Follow-up of #16221
Summary of the issue:
The streamlined AppVeyor build process implemented in #16221, requires documentation in order to be useful.
Description of user facing changes
None.
Description of development approach
• Wrote a document in projectDocs/dev/buildingNVDAOnAppVeyor.md.
• Added references in readme.md and contributing.md, at @seanbudd's suggestion.
• Took the liberty of adding a missing lead heading in buildingNVDA.md.