Clarify that memoise should only apply to pure function?

[dracodoc]

This is a suggestion on package readme and vignette. I found it may not be obvious for some users that memoise is supposed to only be used on pure functions.

I searched stackoverflow for a question I have about memoise, and I found several of them are trying to memoise database inquiries, which are not right in concept.

Maybe the first line of readme should stress about the pure function and point users to the pure function part of advanced R book?

[jimhester]

memoising a database query is fine if you understand doing so assumes the database will always return the same results during the runtime of your program.