add requirements section

[Sakurann]

For example, it took me a while to locate requirements for the identifiers (quoted below) - it would be much cleaner if there is a separate section summarizing all the requirements in the beginning of the document.

This specification allows the use of an HTTP URL (e.g. https://1edtech.org/keys/1) or a DID URL (e.g. did:key:123), but only requires HTTP URL support.

[xaviaracil]

Don't know if the spec file is the right place for this section, or it may fit better in the Implementation Guide.

[xaviaracil]

The implementation guide has already a section about requirements