gitignore .xlsx files

e.g. the spreadsheet of registered students
Add links re: writing data for computers
Merge branch 'gh-pages' of https://github.com/jennybc/2014-05-12-ubc into gh-pages

# By Andy Leung
# Via Andy Leung
* 'gh-pages' of https://github.com/jennybc/2014-05-12-ubc:
  Add line for session06_Rfunctions.html
  Add Rmd file for session 6 R functions