[repo] Create a writing style

[andrewnicols]

We should look at using https://github.com/errata-ai/vale, and there are some great writing guides which we may want to adopt. These are detailed at https://github.com/errata-ai/styles

I'd suggest we look at one or more of:

• Alex as a Vale ruleset
• Proselint as a Vale ruleset
• Microsoft
• Google
• Write-good
• Readability

This is related to #48 but they have different remits.