Be clearer about where {{ post }} and friends are available.
Serve 404 on missing files for serif dev.

This avoids exceptions that lead to 500 errors that result in a
stacktrace. For `serif new` sites, favicon.ico is missing but this
doesn't stop content from actually being up correctly. A potential
source of confusion.

See #22.
Merge pull request #22 from aprescott/dev/serve-404s

`serif dev` server should serve up 404s
Warn about editing _config.yml after `serif new`.
Mention #23 in CHANGELOG.
Merge pull request #23 from aprescott/new/warn-about-password

Warn about editing _config.yml after `serif new`
Mention 404 handling for dev server (#22) in CHANGELOG.
Merge branch 'master' into private-draft-previews

Conflicts:
	CHANGELOG.md