Update help message in README.md, and clarify runtests arguments (#50)

Author: giordano

README.md

@@ -4,18 +4,24 @@ Simple parallel test runner for Julia tests with autodiscovery.
 
 ## Usage
 
-```julia
-julia> import Pkg
+The main entry point of `ParallelTestRunner` is the exported function `runtests`, which takes two mandatory arguments:
+
+* the module for which you want to run the tests
+* the list of arguments passed to the test script, as a `Vector{String}`, this will typically be [`Base.ARGS`](https://docs.julialang.org/en/v1/base/constants/#Base.ARGS).
 
-julia> Pkg.test("ParallelTestRunner"; test_args=["--help"])
+With the `--help` flag you can print a help message:
+
+```julia
+julia> using ParallelTestRunner
 
+julia> ParallelTestRunner.runtests(ParallelTestRunner, ["--help"])
 Usage: runtests.jl [--help] [--list] [--jobs=N] [TESTS...]
 
    --help             Show this text.
    --list             List all available tests.
    --verbose          Print more information during testing.
    --quickfail        Fail the entire run as soon as a single test errored.
-   --jobs=N           Launch `N` processes to perform tests (default: Sys.CPU_THREADS).
+   --jobs=N           Launch `N` processes to perform tests.
 
    Remaining arguments filter the tests that will be executed.
 ```