Add entry for Vale rule: allow.yml

[daobrien]

When we create rules in Vale we need to make sure that they're based on solid guidance, and that we can refer writers to this guidance for more info.

allow.yml tests for "allows" and "permits" and suggests using "enables" or similar instead. Other options are available of course, such as supports, provides, lets, facilitates, etc. This is too much info for the Vale message, but we should include it in the style guide.

Ideally, we should also ensure that these entries have IDs so that we can link directly to them in Vale rules and also in editor snippet entries.