Microsite Enhancements

[juanpedromoreno]

This PR brings some microsite improvements:

• Proper attributions in the global footer, where, from now on, it'll show Github4s is designed and developed by Github4s contributors pointing to the AUTHORS file.
• Extra explanation saying g4s is compatible for both Scala JVM/JS and Scala 2.10, 2.11 and 2.12.
• Adds changelog as link, which might be handy. It has been placed side to the Contributing section.

[juanpedromoreno]

Wait until #144 is merged.

[codecov-io]

Codecov Report

Merging #143 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #143   +/-   ##
=====================================
  Coverage      88%    88%           
=====================================
  Files          36     36           
  Lines         550    550           
  Branches        2      1    -1     
=====================================
  Hits          484    484           
  Misses         66     66

Impacted Files	Coverage Δ
...c/main/scala/github4s/free/domain/Repository.scala	100% <ø> (ø)	⬆️
.../main/scala/github4s/free/domain/PullRequest.scala	100% <ø> (ø)	⬆️
...ub4s/shared/src/main/scala/github4s/Decoders.scala	100% <100%> (ø)	⬆️

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