Document "artifact rule" rationale

The [recent discussion](https://github.com/in-toto/docs/pull/2#discussion_r143289526) about whether and why we decided to replace the implicit `DISALLOW *` with an implicit `ALLOW *` when verifying expected materials and products makes it clear that we need to document these decisions more thoroughly.

in-toto/in-toto#43 shows how the current design of `MATCH` rules (only) evolved.

@SantiagoTorres suggests to create a Wiki that summarizes such on- and offline discussions and provides additional information about the rationale behind certain design decision that would go beyond the scope of the specification.

Read the linked discussion in docs/pull/2 and in-toto/in-toto#43 first to understand the artifact-rule decisions and the evolution of MATCH rules. Review the specification repository's existing documentation structure, then propose where the rationale belongs and document the agreed decisions, including the implicit ALLOW * behavior and related discussions.

Written by the indexing model from the issue text.