Update README and vignette to reflect installation via github instead of cran

[svteichman]

replace mentions of the CRAN version of corncob and install.packages("corncob") with the Github version and remotes::install_github("statdivlab/corncob")

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b32d6b2) 94.12% compared to head (db8c431) 94.12%.

❗ Current head db8c431 differs from pull request most recent head 0e12e48. Consider uploading reports for the commit 0e12e48 to get more accurate results

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #169   +/-   ##
=======================================
  Coverage   94.12%   94.12%           
=======================================
  Files          34       34           
  Lines        1158     1158           
=======================================
  Hits         1090     1090           
  Misses         68       68           

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