Add a clarifying statement at the starting

The statement makes it clear that each and every file should have a
comment header that contains copyright and licensing information.

Suggested-by: Matija Šuklje <[email protected]>
Co-authored-by: Matija Šuklje <[email protected]>
Acked-by: Max Mehl <[email protected]>

Author: carmenbianca

spec/spec.md

@@ -8,8 +8,10 @@ SPDX-License-Identifier: CC-BY-SA-4.0
 
 This specification defines a standardized method for declaring copyright and
 licensing for software projects. The goal of the specification is to have
-unambiguous, machine-readable copyright and licensing information for each
-individual file in a project.
+unambiguous, human- and machine-readable copyright and licensing information for
+each individual file in a project. Ideally this information is embedded into
+every file, so that the information is preserved when the file is copied and
+reused by third parties.
 
 This specification implements [IETF RFC 2119: Key words for use in RFCs to
 Indicate Requirement Levels](https://tools.ietf.org/html/rfc2119).