add section on data constraints
attempt to deal with spacing issue
more space fiddling
minor wording change
Merge branch 'master' of https://github.com/adlnet/xAPI-Spec
Merge branch 'master' into dataconstraints
Rearrange LRS requirements related to API parameters for clarity and applicability
fix typo in header name
Merge pull request #176 from fugu13/dataconstraints

Add a section on data constraints