Invalid links in CONTRIBUTING.md

[phil-davis]

Bugs

https://github.com/librariesio/libraries.io/blob/master/.github/CONTRIBUTING.md has some links that are not valid:

"mission and strategy" points to https://github.com/librariesio/libraries.io/blob/master/strategy.md which does not exist

"personas" points to https://github.com/librariesio/libraries.io/blob/master/personas.md which does not exist

"contributor code of conduct " https://github.com/librariesio/libraries.io/blob/master/.github/CODE_OF_CONDUCT.md (actually it is up one level in the repo root)

"our strategy" points to https://github.com/librariesio/libraries.io/blob/master/.github/strategy.md which does not exist

"labelling policy" points to https://github.com/librariesio/libraries.io/blob/master/labelling.md which does not exist

"visual design" https://github.com/librariesio/libraries.io/issues/labels/visual%20design does not exist

"strategy" points to https://github.com/librariesio/libraries.io/blob/master/strategy.md which does not exist

"overview" points to https://github.com/librariesio/libraries.io/blob/master/overview.md which does not exist

"Slack" points to http://slack.io/ which does not exist

"core team" points to https://github.com/orgs/librariesio/teams/core which does not exist (for me - maybe that list is protected) "deployers" team link has a similar problem.

"filing bugs, requesting enhancements and features" points to https://github.com/librariesio/libraries.io/blob/master/issue_template.md (which is nowadays in .github)

"issuing pull-requests" points to https://github.com/librariesio/libraries.io/blob/master/pull_request_template.md (which is nowadays in .github)

"chat channel on Slack" links to https://github.com/librariesio/libraries.io/blob/master/.github/slack.libraries.io which is not valid.

[phil-davis]

A few of these have obvious fixes. But for a lot of them, I guess that they point to things that existing in the past but do not exist any more.