Apply the recommended formatting rules from '.dir-locals.el' to the
examples in the manual in order to fulfill it's purpose of being a
reference.

* doc/package-hello.scm: Indent it.
* doc/guix.texi (Defining Packages, Version Numbers): Likewise.
* gnu/packages/base.scm (hello): Be consistent with the manual.

* guix/scripts/package.scm (show-help): Add help text for --install-from-file
  option.
  (%options): Add --install-from-file option.
* tests/guix-package.sh: Test it.
* doc/guix.texi ("invoking guix package"): Document it.
* doc/package-hello.scm: New file.
* doc.am (EXTRA_DIST): Add it.