Document the less-absolute-junk parts of DCC

[DanielOaks]

Now we have a CTCP doc. Hooray!

DCC is also fairly widely used so we should probably look at having a document that covers that in some way.

Rough initial references:

• https://web.archive.org/web/20080907101746/http://www.invlogic.com/irc/ctcp2_5.html
• http://patrick.cloke.us/posts/2011/03/08/so-called-irc-specifications/#id6
• http://www.irchelp.org/protocol/dccspec.html

Were these ever actually implemented?

• https://tools.ietf.org/html/draft-smith-irc-dcc2-negotiation-00
• https://bitbucket.org/clokep/irc-docs/raw/default/draft-smith-irc-dcc2-files-00.txt

[DanielOaks]

Will be working on this over in the dcc branch