Add a few more notes on inclusivity and group culture

The wording was borrowed liberally (with tweaks for overlaps with what we
already said) from what @rjbs wrote for ABE.pm:

http://abe.pm.org/
Remove outdated link to YAPC::EU 2013

Sadly, there doesn't yet seem to be a website for YAPC::EU 2014, due to be
held in Sofia.
Add link to LPW 2013