Add society info to the homepage

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We should add infos on what we do, our issue trackers (this one and the sps17 one), our contact info and how to join us. Anything else?

[chiesax]

• link to association documents
• info on how to become members (including fee)
• info on how to become sponsors

[dbrgn]

• info on what a membership means (helping to organize the conference)

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I reworked the call for helpers page to include some information about the society: http://www.python-summit.ch/pages/call-for-helpers.html

Basically dividing the helpers into one-day and long-term helpers. The sponsoring, once defined, should go to a separate page in my opinion.