Add link to #rust-sci in README

[jturner314]

It's helpful to have a place for users to chat. The link is to #rust-sci in the Mibbit client (the same client used for the "Mozilla IRC" link on https://www.rust-lang.org/community). Mibbit is nice because it provides a way for people to join the chat using their browser without even having to register.

[jturner314]

The Mozilla IRC server is shutting down in the near future, so we'll need to choose something other than #rust-sci on irc.mozilla.org. It looks like #rust-sci is migrating to oftc.net (link to web chat), but let's wait a little while for things to stabilize before adding it to the README.

[ZuseZ4]

@bluss Shall we merge this?
It's not a breaking change, but I see no reason to keep it open, since it's just a doc change.

[bluss]

That channel has moved to matrix entirely, so I guess we'd like to link to matrix.

[jturner314]

I've changed this PR to provide a link to the Matrix room and the bridged IRC room on OFTC. (It's useful to provide a link to the IRC web chat so that users who don't want to go to the effort of creating a Matrix account can still ask a quick question.) Click here to preview how GitHub renders the proposed README.