Document the SASL DH-BLOWFISH and DH-AES mechanisms

The files will have to be moved and/or renamed before merging, but for review having Markdown rendered will be better.

 - SASL [DH-BLOWFISH](https://github.com/grawity/ircv3-specifications/blob/sasl-dh-mechs/sasl-dh-blowfish.md)
 - SASL [DH-AES](https://github.com/grawity/ircv3-specifications/blob/sasl-dh-mechs/sasl-dh-aes.md)

Since this is like the first time I have to document binary formats and cryptographic protocols, comments are welcome.