Fix errors in and update README; Adjust bad data type in example text

This fixes some errors in the README and adds Linux installation instructions (I hope you find it unobtrusive). The second commit is a rebase of what was once two commits- the example in those exercises asks the student to submit an answer within 1/2...but the grader only accepts decimals. The commit changes the example text to a decimal, and move it some so as to avoid ending the sentence with a integer.