Added limitations on numeric qtypes

[exprin]

I added the following:

• Explanation of limitations of number and decimal question types
• How to add number, range, and decimal question types in XLSForm
• a downloadable XLSForm

@joshuaberetta kindly review

[exprin]

@exprin, is there a reason why the two files:

• source/images/pulling_data_excelquery/excel_data_source_settings.gif
• source/images/pulling_data_excelquery/excel_load_table.gif Are included in this PR?

I might have accidentally added the "Pulling data into Power Query" files into master instead of creating a new branch. SO now those files are moving with me to every branch. I must have added them to the commit for this branch accidentally

[exprin]

@joshuaberetta I have sent a commit to this branch resolving the comments