Add IRC channel to get involved page

In response to #16 (closed), I added information about the IRC channel to the 'get involved' page.

I also collected info on how to join via matrix, however I don't know any way to succinctly express how to join and I'm not sure a lenghty explanation about matrix is something we want to have on that page.

Anyway I'm putting my findings here, so that we might add it in the future:

• the channel name is #freenode_#briar:matrix.org
• apparently there is also an internal id that some clients might need to join: !DSFhcVWSWRGSBGWzXy:matrix.org
• from the element.io app, it is possible to join the bridge channel by typing /join #freenode_#briar:matrix.org inside any channel