Update README.md

* be more verbose in some contexts:
  * netrc file
  * 'session keyfile' generation
* update warning about other files on the ftp storage to
  reflect the committed changes to the script
* give the license information its own headline
Create a "better" example RSA key

* increase example keylength from 4096 to 8192
* change example encryption scheme from 3DES to AES or Camellia
* add some boilerplate about:
  * passphrases
  * restore process handling
  * not to rely on other people to think