title: java sign public pgp key with bouncycastle
body: <p>I've got a doubt..
  I have to sign a pgp public key using bouncycastle api supposedly.
  Now: to my understanding signing a key with another means ultimately adding to this public key a "certificate".
  Thus lacking any other way, I've gone blind searching in the library. 
  my only find so far has been  method generateCertification inside PGPSignatureGenerator. But this method generate a certification between a master PgpPublicKey and another PgpPublicKey.. And this strikes me as strange:
  I assumed that in order to trust another public key, that has to be signed with your own private pgp key just like in regular x.509 with CA certification in a manner..
  This was assumption by some methods that I saw when trying to get some ideas from other library: didisoft for example has a similar method on a keystore where you have to provide the PgpPrivatekey keyuid...</p>
  
  <p>Anyone has any hint or a piece of code to propose?
  Thanks in advance.</p>