Adds small clarifications to schema.md for complete beginners

[Kaimura]

Just adding 2 small adjustments to clarify what to do and not to do because I or my colleagues ran into these issues. Especially the 'adding a mutation result' part to update the cache and UI more easily! If You want you can switch from bold text to cursive text since it is unusual to have a whole sentence in bold. Anyhow I think that parts really needs to be pointed out - people shouldn't think it is just some optional fun since the cache problems that occur by leaving this out can be too stressfull.