Adding a link to CSSWG Services section

[astearns]

I’d like to see the Services section for the CSSWG include this link, which is currently the best introduction we have for new members:

Introduction to CSSWG process: https://fantasai.inkedblade.net/weblog/2011/inside-csswg/

I started poking around to see where I could add this, and it wasn’t immediately obvious.

[plehegar]

https://github.com/w3c/onboarding contains the templates used when someone new joins a Group. Those could be tailored as needed.

Group have 2 public homepages:

• the default ones: intended for a large public audience. We do require consistency among those.
• the group ones: linked from the default homepage, they contain information that could be tailored at will.

I suggest adding the link to the CSSWG group page, as well as the onboarding email.

[plehegar]

@astearns @svgeesus , feel free send a PR to change the CSSWG onboarding email (and assign it to me if you can't merge)

[plehegar]

CSS onboarding template

[plehegar]

fyi @fantasai