Improve instructions for getting coverage with Pkg.test (#248)

* Improve instructions

* Update README.md

README.md

@@ -22,7 +22,7 @@ julia --code-coverage=user
 julia --code-coverage=tracefile-%p.info --code-coverage=user  # available in Julia v1.1+
 ```
 
-*Step 2:* Run your tests (e.g., `include("runtests.jl")`) and quit Julia.
+*Step 2:* Run your tests (e.g., `include("runtests.jl")`) and quit Julia. (If you use `Pkg.test` to run your tests, set the `coverage` keyword argument to `true`, i.e. `Pkg.test("MyPkg"; coverage=true)`.)
 
 *Step 3:* Navigate to the top-level directory of your package, restart Julia (with no special flags) and analyze your code coverage: