`devtools::check()` results in 1 ERROR, 3 WARNINGs, 2 NOTEs

[ttimbers]

I ran devtools::check() on your package and received the following errors, warnings and notes. Please address these.

Status: 1 ERROR, 3 WARNINGs, 2 NOTEs
See
  ‘/private/var/folders/9w/mzjb0_j118vd2k4762ggh43w0000gn/T/Rtmp1WG7fM/CrossR.Rcheck/00check.log’
for details.

R CMD check results
1 error  | 3 warnings | 2 notes
checking examples ... ERROR
Running examples in ‘CrossR-Ex.R’ failed
The error most likely occurred in:

> base::assign(".ptime", proc.time(), pos = "CheckExEnv")
> ### Name: cross_validation
> ### Title: cross_validation: Implement k-fold cross validation, with
> ###   specified k, returning the scores for each fold.
> ### Aliases: cross_validation
> 
> ### ** Examples
> 
> cross_validation(model = lm, X = X_iris, y = y_iris, k = 5)
Error in cross_validation(model = lm, X = X_iris, y = y_iris, k = 5) : 
  unused argument (model = lm)
Execution halted

checking DESCRIPTION meta-information ... WARNING
Non-standard license specification:
  What license is it under?
Standardizable: FALSE

checking Rd \usage sections ... WARNING
Undocumented arguments in documentation object 'cross_validation'
  ‘X’ ‘y’ ‘k’ ‘shuffle’ ‘random_state’
Documented arguments not in \usage in documentation object 'cross_validation':
  ‘X:’ ‘y:’ ‘k:’ ‘shuffle:’ ‘random_state:’

Functions with \usage entries need to have the appropriate \alias
entries, and all their arguments documented.
The \usage entries must correspond to syntactically valid R code.
See chapter ‘Writing R documentation files’ in the ‘Writing R
Extensions’ manual.

checking for unstated dependencies in ‘tests’ ... WARNING
'library' or 'require' call not declared from: ‘testthat’

checking top-level files ... NOTE
File
  LICENSE
is not mentioned in the DESCRIPTION file.
Non-standard files/directories found at top level:
  ‘CONDUCT.md’ ‘CONTRIBUTING.md’

checking R code for possible problems ... NOTE
cross_validation: no visible global function definition for ‘predict’
cross_validation: no visible global function definition for ‘cor’
summary_cv: no visible global function definition for ‘median’
summary_cv: no visible global function definition for ‘sd’
train_test_split: no visible global function definition for
  ‘na.exclude’
Undefined global functions or variables:
  cor median na.exclude predict sd
Consider adding
  importFrom("stats", "cor", "median", "na.exclude", "predict", "sd")
to your NAMESPACE file.