Add residuals convenience function to docs

[ym-han]

Preliminary attempt at adding discussion of residuals convenience function to docs, since it's in the code (and obviously in StatsBase), but not, as far as I can tell, mentioned otherwise anywhere in any of the docs.

[ym-han]

This is my first real PR to a Julia package repo, so please forgive me if I've made any mistakes in setting this up.

[ym-han]

Sorry about taking so long to get to this; I got busy with school. Let me know if there's anything else you would like me to add or fix.

[nalimilan]

Something seems to have gone wrong when rebasing: many unrelated changes appear in the diff, and there are conflicts.

[ym-han]

Sorry about that; I'll fix it by tomorrow night.

[ym-han]

OK this should be fixed / be what we want now

[codecov-commenter]

Codecov Report

Merging #445 (69cb024) into master (64ec692) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #445   +/-   ##
=======================================
  Coverage   84.01%   84.01%           
=======================================
  Files           7        7           
  Lines         807      807           
=======================================
  Hits          678      678           
  Misses        129      129           

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[nalimilan]

Thanks!