Add ability to keep local cache for debmirror's repo syncs

Add new cache_dir parameter to keep local cache for debmirror
repo and re-use it. During first run debmirror will create
cache and then using hardlinks cache will be copied to target
repo. All next runs will copy cache first and then run debmirror
with prepared cached.

Change-Id: Ib7932e64c75743b06b3b12507fe87a30d4d93311
(cherry picked from commit 964ba9a2b45335c2c7cc2bcf7aab094296de0d36)
diff --git a/README.rst b/README.rst
index 4c1b825..dde2942 100644
--- a/README.rst
+++ b/README.rst
@@ -30,6 +30,7 @@
               arch: [ 'amd64' ]
               mirror_host: "mirror.mirantis.com" # rsync
               mirror_root: ':mirror/nightly/ubuntu/'
+              cache_dir: "/var/www/mirror/.cache/ubuntu"
               target_dir: "/var/www/mirror/ubuntu/"
               log_file: "/var/www/mirror/target01_log.log"
               dist: [ xenial ] #, xenial-security, xenial-updates ]
@@ -47,6 +48,9 @@
                 08: "--include='/main(.*)python-(.*)doc'"
                 09: "--include='/main(.*)python-(.*)network'"
 
+Parameter cache_dir is optional and can be used to avoid extra disk space
+usage for repos, which can have same packages, by using hardlinks to files.
+
 **Documentation and bugs**
 
 * http://salt-formulas.readthedocs.io/
@@ -71,3 +75,4 @@
 * #salt-formulas @ irc.freenode.net
    Use this IRC channel in case of any questions or feedback which is always
    welcome
+