Update README.rdoc

Maintenance mode note in readme.

Author: delano

README.rdoc

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 = Net::SFTP
 
+<em><b>Please note: this project is in maintenance mode. It is not under active development but pull requests are very much welcome. Just be sure to include tests! -- delano</b></em>
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 * Docs: http://net-ssh.github.com/net-sftp
 * Issues: https://github.com/net-ssh/net-sftp/issues
 * Codes: https://github.com/net-ssh/net-sftp