Licensing, How to apply: Add notes about `SPDX-File` and `SPDX-Snippet`

[andreashaerter]

REUSE is using SPDX tags for the inline comments and .license files. It is good to know how they work. A few examples should be added to our guide, especially handling:

• SPDX-FileCopyrightText: Can be used multiple times in a row, cf. https://spdx.github.io/spdx-spec/v2.3/file-tags/#h2-file-tags-format
• SPDX-SPDX-FileContributor: Exists, quite useful if a snippet or file was copied form another project but only changed in a minimal way to fit into another context.
• SPDX-Snippet exists, cf. https://spdx.github.io/spdx-spec/v2.3/file-tags/#h3-snippet-tags-format

  SPDX-License-Identifier: MIT
  SPDX-SnippetCopyrightText: 2022 Jane Doe <jane@example.com>
  ...
  SPDX-SnippetEnd