remove mention of for loop

Couldn't see a mention of for loops, perhaps it was in an earlier version, or yet to appear?
Update 01-starting-with-data.Rmd
Update 01-starting-with-data.Rmd
Update 01-starting-with-data.Rmd
Update 01-starting-with-data.Rmd
added excel lesson directory
Added CONTRIBUTING.md file for information on how to contribute to the repo
moved files from data into data/biology so there can be other datasets
added directory for python materials
Create ecology_spreadsheets
Merge pull request #6 from cbahlai/master

Create ecology_spreadsheets
added md extension to Markdown file
Merge pull request #3 from benmarwick/master

remove mention of for loop
Update ecology_spreadsheets.md
Merge pull request #7 from cbahlai/patch-2

Update ecology_spreadsheets.md
Started working on examples with bio data
Merging Christie's file ecology_spreadsheets.md
Splitted lessons into files and put them into eco-examples directory
Moved img to main directory and added bad_null example image
Added file to img and updated wording