Update config and contributing guide
This addresses some issues with config
and CONTRIBUTING.md
encountered while walking davigpgil through getting set up for Trisquel development
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While running package helpers, signing keys fail to import from keyserver.ubuntu.com on Flidas (for both chaosmonk and davidpgil)
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The link to trisquel-packages in the contributing guide is easy to miss
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Some information in the contributing guide is not presented in the ideal order for understanding what's going on