README: use CamelCase for option names

Git option names are case-insensitive. CamelCasing (Capital on first
letter of each word) makes them more readable without changing the
meaning. This is the convention in "man git-config" already.
README: say explicitly that "reference change" emails are called "refchange"
README: Mention announce emails in the introduction
Fix indentation of paragraphs within enumerated list.
Merge branch 'doc'

The list of config options starts being a bit long.

This branch tries to make it more readable and easier to go through,
by making the link between the 1. / 2. (and now 3.) enumeration at the
top and the list of options.

Perhaps it's time to introduce sections/subsections in the list too,
but I didn't do that.