Write some documentation about use of colours

[chris48s]

📋 Description

We've had a couple of PRs recently where contributors have wanted to contribute badges using branded colours in preference to the standard palette.

We have some pretty good docs we can point to when it comes to thing like conventions for badge URLs but I'd like to have some guidance written down about use of colours.

I need to have a think about exactly what to write, but the key point is basically: We use colour to convey information and ensure consistency, not to promote a project or brand. There may also be a few places where we need to pull some older services into line with this as part of writing the guidance.

This is related to shields values of being semantic and non-promotional