[Suggestion] Add more details about YAP PKGBUILD spec file

[M0Rf30]

Is your feature request related to a problem? Please describe.
We need to extend some details about the PKGBUILD rules

Describe the solution you'd like
Add details about:

• the backup path (absolute or relative?)
• highligh differences between yap and arch PKGBUILD

Describe alternatives you've considered

• remove the differences: parsing directly arch PKGBUILD, move to another filename (these are breaking changes)

[M0Rf30]

In these days I deeply think about extreme semplification of the overall logic.
Questions like:

• how to manage the spec definition?
• does it make sense to be non-compliant with Arch PKGBUILD?
• how to rewrite an efficient parser without going crazy?
• which token to use to determine different distro scopes? (nowadays we use :)

and the answer is the obvious and rock-solid: Keep it simple stupid and let's parse simple BASH spec files, making yap fully compliant with Arch PKGBUILD

This consideration answers to the first 3 questions in a very simple way:

• yap PKGBUILD == Arch PKGBUILD
• no
• plenty of high quality pure-go bash parser out there
• double underscores __, doesn't break syntax checkers or shell linters