Link between this and related repos

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link to AND from

• [x] this repo https://github.com/ssbc/ssb-tribes2/commit/e3ce29289001f0089e012394514b9b989b6e2baf
• [x] demo https://github.com/ssbc/ssb-tribes2-demo/ https://github.com/ssbc/ssb-tribes2-demo/commit/282c5d6685588ba91e1ea988771aef44046ad647
• [x] spec https://github.com/ssbc/private-group-spec https://github.com/ssbc/private-group-spec/commit/47d2e17298baca0c2cb6bf1abce352562a3f6869
• [x] exclusion spec https://github.com/ssbc/ssb-group-exclusion-spec https://github.com/ssbc/ssb-group-exclusion-spec/pull/33

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@mixmix since it was just a small readme change, i pushed straight to master (except for exclude which andre 'owns')