Provide some guidance on spelling and checking spelling

The consistency of spelling on the Apertis website isn't great. We use UK
English in places and US English in others. This could get problematic
when users are looking for documents describing artifacts when we've spelt
it "artefacts" in some relevant documents.

Let's start the process by documenting the use of the Aspell spell
checker. Start building a custom dictionary of acceptable terms, with the
plan of integrating this into the CI/CD process at some point in the
future.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>