Add to commit guide: empty values on datatypes are invalid

[ppazos]

1. the commit will return errors on those fields
2. add examples of possible empty values for text, coded text and quantity
3. suggest that if empty values need to be added, they should use ELEMENT.null_flavour instead of ELEMENT.value

[ppazos]

guide updated on https://github.com/ppazos/cabolabs-ehrserver/commit/88892fd94a7fb7771a7325c220e0f76ccd427b05