Update 'if.md' to note using the plus. (#3037)
Update 'if.md' to note using the plus. (#3037)


Microsoft/pxt,playi/pxt,switch-education/pxt,switchinnovations/pxt,Microsoft/pxt,playi/pxt,Microsoft/pxt,switch-education/pxt,Microsoft/pxt,Microsoft/pxt,switchinnovations/pxt,switch-education/pxt,switchinnovations/pxt,playi/pxt,switch-education/pxt,playi/pxt,switchinnovations/pxt,switch-education/pxt