doc: Explain how to prepare a release candidate

Add the explanation to a new file 'doc/how-to-maintain-waffle.asciidoc'.

Fixes #2: https://github.com/waffle-gl/waffle/issues/2
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>
doc: Add release notes for Waffle 1.4.0-rc1

This patch reformats and cleans up the placeholder file
'waffle-next.txt', renaming it to 'waffle-1.4.0.txt'.

Signed-off-by: Chad Versace <chad.versace@linux.intel.com>