Help/FAQ: trial inclusion in search

[benmeg]

During user testing, users have commented on discrepancies between the number of results from OpenTrials vs. ClinicalTrials.gov (OpenTrials tends to have fewwe) - notwithstanding these differences (this is being document here: #607) the users have asked what the inclusion criteria is for trials to be on OpenTrials.

Perhaps a short help section can be written to explain this (and explain the current differences/beta state/etc).

This is related to #761 where we aim to explain search functionality