Clarify rules on file termination

Since 17th Jun 2021, these points have been agreed:

• Every line must be terminated by a Unix line feed character (LF, decimal 10, hexadecimal 0x0A).
• Carriage returns (CR, decimal 13, hexadecimal 0x0D) must NOT be used alone or with LFs.
• There must be no whitespace characters (spaces or tabs) at the end of any line.
• There must be no extra blank lines at the end of a file; every file should end with a single LF character.

Note: This is consistent with the conventions of PSR-12.