Proposal: Allow frontmatter in index and log files.

The v0.1 spec states:

> Index files contain no frontmatter.

As far as I can see, there is no reason why the OKF standard _needs_ to explicitly disallow frontmatter in index files. And conversely, it creates an unnecessary obstacle for cross-compatibility with other standards or tools, which may have their own uses for frontmatter.

The same applies to log files, for which the spec makes no mention of frontmatter either way.

I propose explicitly allowing frontmatter in both `index.md`, and `log.md`, while making clear that it is not _required_ in either as it is in concept documents.