chore: update packages to deeplink into their respective directories

[quantizor]

I think this makes slightly more sense than just linking to root. The visitor can always back up if they want to.

This might also be a good time to start thinking about individual package READMEs. The NPM pages are a little bare without them: https://www.npmjs.com/package/react-emotion

[codecov-io]

Codecov Report

Merging #276 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #276   +/-   ##
=======================================
  Coverage   90.37%   90.37%           
=======================================
  Files          21       21           
  Lines         966      966           
  Branches      260      260           
=======================================
  Hits          873      873           
  Misses         75       75           
  Partials       18       18

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[emmatown]

I'm not sure but I think this could break the links to Issues and PRs on npm.

[quantizor]

I'm not sure about pulls, but I think issues uses a different property: https://github.com/tkh44/emotion/pull/276/files#diff-54043be09c6d011d84adfabf6e46686aR34

[tkh44]

If we are removing the git reference we should just use this format:

https://github.com/babel/babel/blob/7.0/packages/babel-plugin-transform-decorators/package.json#L7

[tkh44]

Thanks!