Add ssh-ca

Certificate based SSH allows us to launch a server at time X and grant
SSH access to that server later at time X + Y without touching the
authorized keys file. Further it allows us to generate certificates that
expire at some predefined time meaning that users can be granted access
to a system for a short period of time.

README.md

@@ -342,6 +342,7 @@ Comparison of NoSQL servers: http://kkovacs.eu/cassandra-vs-mongodb-vs-couchdb-v
 * [Mosh](http://mosh.mit.edu/) - The mobile shell.
 * [parallel-ssh](http://code.google.com/p/parallel-ssh/) - Provides parallel versions of OpenSSH and related tools.
 * [SSH Power Tool](http://code.google.com/p/sshpt/) - Execute commands and upload files to many servers simultaneously without using pre-shared keys.
+* [ssh-ca](https://github.com/cloudtools/ssh-ca/) - Certificate based SSH allows us to launch a server at time X and grant SSH access to that server later at time X + Y without touching the authorized keys file. Further it allows us to generate certificates that expire at some predefined time meaning that users can be granted access to a system for a short period of time.
 
 ## Statistics
 *Analytics software.*