Add warnings to README and --help about syncing private keys.

This should really become non-default behavior, per lra#109/lra#512, but in
the meantime let's at least warn users who might not like it.

README.md

@@ -209,7 +209,7 @@ See the [README](doc/README.md) file in the doc directory for more info.
   - [Gmail Notifr](http://ashchan.com/projects/gmail-notifr)
   - [GMVault](http://gmvault.org/)
   - [Go2Shell](http://zipzapmac.com/Go2Shell)
-  - [GnuPG](http://www.gnupg.org/)
+  - [GnuPG](http://www.gnupg.org/)  (NOTE: includes private keys)
   - [GrandTotal 3](http://www.mediaatelier.com/GrandTotal3/)
   - [Gradle](http://gradle.org)
   - [Gnome SSH Tunnel Manager](http://sourceforge.net/projects/gstm/)
@@ -276,7 +276,7 @@ See the [README](doc/README.md) file in the doc directory for more info.
   - [Oh My Zsh](https://github.com/robbyrussell/oh-my-zsh)
   - [OmniFocus](http://www.omnigroup.com/products/omnifocus/)
   - [OmniGraffle](http://www.omnigroup.com/omnigraffle/)
-  - [OpenSSH](http://www.openssh.org/)
+  - [OpenSSH](http://www.openssh.org/) (NOTE: includes private keys)
   - [Pass](http://www.passwordstore.org/)
   - [Pastebot](http://tapbots.com/software/pastebot/)
   - [Path Finder](http://www.cocoatech.com/pathfinder/)

mackup/main.py

@@ -18,11 +18,18 @@
 Modes of action:
  1. list: display a list of all supported applications.
  2. backup: sync your conf files to your synced storage, use this the 1st time
-    you use Mackup.
+    you use Mackup. (Note that by default this will sync private keys used by
+    OpenSSH and GnuPG.)
  3. restore: link the conf files already in your synced storage on your system,
     use it on any new system you use.
  4. uninstall: reset everything as it was before using Mackup.
 
+By default, Mackup syncs all application data (including private keys!) via
+Dropbox, but may be configured to exclude applications or use a different
+backend with a .mackup.cfg file.
+
+See https://github.com/lra/mackup/tree/master/doc for more information.
+
 """
 from docopt import docopt
 from .appsdb import ApplicationsDatabase