Include design guidelines

[chrisjwelly]

I realise while working on #13 that we may need to come up with a common guideline for design. E.g. If using constraintLayout, how should we apply the margin to the buttons?

It will be a helpful reference when we are designing a new page.

[chrisjwelly]

If we are taking template designs, this may be obsolete. I think it's not worth doing this now.