Update and document relationships between extensions

[zimeon]

Editors propose removing "Obsoletes" and "Obsoleted by" from the headers of extensions, and instead replacing for any cases where there is a relation with a new line:

For 0001:

• Related to 0009-digest-algorithms (obsoleted by)

For 0003 (soon, when #86 approved):

• Related to 0012-hash-and-no-prefix-id-n-tuple-storage-layout (extended by)

and allowing the possibility of multiple relationships:

• Related to nnnn-ext-a (obsoletes), mmmm-ext-b (extended by)

This approach requires an update to the headers section of the repository README. To document the approach and list the set of relationships supported along with their intended use. For now we propose:

• obsoletes - Use when a new extension SHOULD be used in preference to the prior extension. This may be because it corrects errors, or updates a canonical list, for example.
• obsoleted by - Inverse of obsoletes, both MUST be used together.
• extends - Use when a new extension adds but does not remove functionality compared to a prior extension, and there is nothing to suggest that use of the prior extension SHOULD be avoided.
• extended by - Inverse of extends, both MUST be used together.