added link to docs/ and list of existing ones

[himorin]

for #292, #293

Rather than having list in the top level, I've added a new README in docs/ directory. How about this?
We may be easily noticed that we have a list, and this may tell us an action to update README when adding a new directory into docs/.

[xfq]

IMHO linking directly to the documents themselves is more convenient (for the readers of the documents) than linking to these directories, i.e., we can turn line-composition/ into https://w3c.github.io/jlreq/docs/line-composition/.

[himorin]

Actually, somehow thought of putting a link to github.io...
How about to have both, like [title](relative link) ([github.io](link to github.io))??

[r12a]

I prefer to have the links at the top level, not hidden and 2 steps further away. I'm not sure what problem this PR is trying to solve.

[r12a]

My suggestion would be to link from the top README page to the major documents, ie.

• Rules for Simple Placement of Line Composition (idea) / 簡便な行組版ルール（案)
• Design of character classes for inter-character spacing extended to Unicode / Unicode に拡張した字間プロパティの設計
• Analysis of changes required for Adobe-Japan1 (AJ1) Japanese fonts to adopt UAX 50 / AJ!フォントをUAX 50準拠とするのに必要とされる変更点の解析

If we have miscellaneous small items that may be worth listing so that we don't forget where they are, we could add them to the directory README, and point to the directory from the top page (similar to the link you are proposing).

I think it also makes sense to point to the historical document(s) from the directory README rather than the top README (as your PR proposes). I think we don't want to make those particularly visible.

how does that sound?

[kidayasuo]

The change looks good. Approving.