Add minimal example to README

[roosemberth]

Motivation for this MR

Every time I start working in a new Haskell project, I find myself in this
repository's README, copy one of the provided examples, but spend most of the
time removing keys I don't need. I think a simple example one can simply
'copy over' is both useful to making the tool accessible to beginners and
a practical base for experienced developers to build on.

Changes in this MR

This MR adds a short and simple example, matching the default cabal init
'Library and Executable' generation (app and test directories with
Main.hs inside them and library inside src).

[roosemberth]

@sol Gentle ping ^^

[mpilgrem]

Perhaps my-package would be a better dummy name, as this will produce a Cabal file for a package with more than a library component.

[mpilgrem]

I would include spec-version: 0.36.0, to ensure modern behaviour.

[mpilgrem]

What is the basis for these bounds? The generated Cabal file will specify cabal-version: 1.12 which might imply base >= 4.4 (GHC 7.2.1 came with Cabal-1.12.0).

[mpilgrem]

While executable: { ... } is strictly minimal, perhaps executables: { my-exe: { ... } } would be more useful?

[mpilgrem]

Perhaps my-test-suite or my-package-test (given what cabal init would suggest) would be a better dummy name than spec?

[mpilgrem]

For a minimal example, why include src as a source directory for the source code of a test suite application (also given what cabal init would suggest)?

[mpilgrem]

For a minimal example, I would expect a dependency on the package itself but not QuickCheck.

[mpilgrem]

I think, strictly, (if you follow my other suggestions below):

... a package with a main library with source code under directory `src`, an
executable named `my-exe` with source code under `app`, and a test suite named
`my-test-suite` with source code under `test`:

[mpilgrem]

It may be worth acknowledging in the introduction that users of Stack have access to templates for projects that include package.yaml files, but users of Cabal (the tool) may find the minimal example useful.