README: Link to docs repo when appropriate
README: Link to docs repo when appropriate

Some docs have been moved from x-common to docs repository.

* Git Basics
  * https://github.com/exercism/x-common/pull/819
  * https://github.com/exercism/docs/pull/43
* Porting an Exercise
  * https://github.com/exercism/x-common/pull/808
  * https://github.com/exercism/docs/pull/4

exercism/xhaskell