build: Add staging directories for future changes.

This is the first step in changing the release process so that changes
made to the CHANGES and UPGRADE.txt files do not result in merge
conflicts every time multiple people modify these files. The changes
made will go in these new directories: doc/CHANGES-staging and
doc/UPGRADE-staging. The README.md files explain how things will work,
but here's a little overview. When you make a change that would go in
either CHANGES or UPGRADE.txt, this should instead be documented in a
new file in the doc/CHANGES-staging or doc/UPGRADE-staging directory,
respectively. The format will look like this:

   Subject: res_pjsip

   A description that explains the changes made and why. The release
   script will handle the bulleting and section separators! The
   'Subject:' header is case-sensitive.

   You can still separate with new lines within your description.

   Subject: res_ari
   Master-Only: true

   You can have more than one subject, and they don't have to be the
   same! Also, the 'Master-Only' header should always be true and is
   also case-sensitive (but the value is not - you can have 'true' or
   'True'). This header will only ever be present in the master branch.

For more information, check out the wiki page:
https://wiki.asterisk.org/wiki/display/AST/CHANGES+and+UPGRADE.txt

This is an initial change for ASTERISK_28111. Functionally, this will
make no difference, but it will prep the directories for when the
changes from CHANGES and UPGRADE.txt are extracted.

Change-Id: I8d852f284f66ac456b26dcb899ee46babf7d15b6