docs(CONTRIBUTING.md): add link to Running Specs
in the flight manual
MODIFY
CONTRIBUTING.md
CONTRIBUTING.md
See the [Atom contributing guide](https://github.com/atom/atom/blob/master/CONTRIBUTING.md)
+
+To run tests on core atom packages like this one, see: https://flight-manual.atom.io/hacking-atom/sections/writing-specs/#running-specs

docs(CONTRIBUTING.md): add link to Running Specs

in the flight manual