Make contributing page up to date

[laanwj]

• Remove mention of (retired) bitcoin-discuss list
• Remove mention of bitcoin-core-dev list under "discussion", as it has been changed into an announcements list and there was never any discussion there in the first place
• Add mention of other relevant IRC channels

[harding]

This fails tests because translated pages still use the old bitcoin-discuss link:

## Check for broken Markdown reference-style links that are displayed in text unchanged, e.g. [broken][broken link]
! find _site/ -name '*.html' | xargs grep ']\[' | grep -v skip-test | grep .
_site/zh_CN/contribute/index.html:比特币协议讨论区 <a href="http://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev">bitcoin-dev</a> 和一个普通比特币讨论区 [bitcoin-discuss][].</p>
_site/ja/contribute/index.html:<p>Bitcoin Coreに関する議論の大部分は、irc.freenode.netの<a href="https://webchat.freenode.net?channels=%23bitcoin-core-dev">#bitcoin-core-dev</a> IRCチャネルもしくは<a href="http://lists.linuxfoundation.org/mailman/listinfo/bitcoin-core-dev">bitcoin-core-dev</a>メーリングリストで行われます。Bitcoinプロトコルについて議論する<a href="http://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev">bitcoin-dev</a>や一般的なBitcoinの議論をする[bitcoin-discuss][]などのメーリングリストもあります。</p>
Makefile:20: recipe for target 'test-fast' failed

Options are:

1. Try to edit the translated pages
2. Restore the [bitcoin-discuss] link to the references.md file and increment the version header in the 2016-01-01-contribute.md file; this will unbreak the links and let translators know their pages need to be updated.

Sorry for the extra work.

[laanwj]

I don't feel comfortable editing Chinese and Japanese here as it doesn't seem to be clearly separable (like an enumeration item). So went with [2].

[harding]

Tested ACK ebca57c3fd831aab3fdf5b1bc213f4604e55019c . Built a preview, clicked the IRC channel links, and made sure they matched what's in my IRC client. Also reviewed diff. Thanks!