Update README.md with note on imports

[pepijn-devries]

When using functions from other packages using the cpp11 package function, I noticed that when calling such functions R would get stuck and doesn't throw errors or warnings. It took me a while to realise that these functions weren't imported to the namespace. After importing the functions, everything worked smoothly. I've added this note to help others avoid running into the same problem.

[romainfrancois]

Thanks for the contribution. I feel that this is a little too specific for a README. Perhaps we can instead do something here to error when the namespace is not found:

cpp11/inst/include/cpp11/function.hpp

Line 72 in 3c36f7f

return safe[Rf_findVarInFrame](R_NamespaceRegistry, name_sexp);

with a clear error message, and some instructions on how to proceed.

[pepijn-devries]

Thanks for the response. Yes an error-message will definitely help here.