Update NEWS.md

NEWS.md

@@ -11,7 +11,8 @@
     still use the old names for now, but please switch over to using `...Units` instead
     of `...Unit` in this release as the old names will be removed in a future release.
   - `c` is now a unit, to permit converting mass into `MeV/c^2`, for example. `c0` is
-    still a quantity equal to the speed of light in vacuum, in units of `m/s`.
+    still a quantity equal to the speed of light in vacuum, in units of `m/s` 
+    [#67](https://github.com/ajkeller34/Unitful.jl/issues/67).
 - v0.1.5
   - Patch for Julia PR [#20889](https://github.com/JuliaLang/julia/pull/20889), which
     changes how lowering is done for exponentiation of integer literals.