hledger/doc

Miscellaneous project docs that don't fit elsewhere.
See also:

- the project readme in the top directory
- the package changelogs and man pages in hledger-lib/, hledger/ etc.
- the website content in site/

Some doc notes, until a better place is found:

In site/release-notes.md, we stopped mentioning minor releases
around 0.27. The old minor releases should probably be removed
or promoted to the same heading level as major releases.

Changelogs started mentioning committer names from 2017/1,
for hledger-ui-1.1.1 (because they won't appear on the release notes).
Could do it just for minor releases but might as well do it for all.
Could do it for past releases but no pressing need.

Changelogs are plain text, but started including some markdown formatting
from 1.0, for ease of pasting from release notes ? Should make consistent.