Add doctest example

[benjaminweb]

Setup

• Create directory test.
• Create file test/Doctest.hs:

Test.DocTest
main :: IO ()
main = doctest ["-isrc", "src/Main.hs", "src/MyModule.hs"]

• Add to package.yaml:

tests:
    doctest:
      main: Doctests.hs
      source-dirs:
        - test
      dependencies:
        - doctest

Execution

• execute hpack in project root directory.
• run doctests with stack test in root directory.