docs: add changelog for v1.3.0, update authors and version const (gin…

…-gonic#1478)

* docs: add changelog for v1.3.0, update authors and version const

*  add link for every referenced pull request (gin-gonic#1481)

* docs: add changelog for v1.3.0, update authors and version const

* add link for pr

First release candidate for Gin 1.0