Move some old doc files under doc/

ChangeLog.pre-git-import is useless since the Git log contains the
pre-Git history anyway.

AUTHORS is not really useful, as many more people have been committing
to Clutter for a while now.

diff --git a/AUTHORS b/doc/AUTHORS.old
similarity index 100%
rename from AUTHORS
rename to doc/AUTHORS.old

diff --git a/ChangeLog.pre-git-import b/doc/ChangeLog.pre-git-import
similarity index 100%
rename from ChangeLog.pre-git-import
rename to doc/ChangeLog.pre-git-import