Clarify the definition of "required"

[ablack3]

CDM fields are marked as required or not but it isn't obvious what exactly it means for a field to be required. This PR adds a sentence that makes is clear that non-required fields must still be present in the database but do not need to be populated with data.

#535

OHDSI/OpenSourceCommunity#8

[MaximMoinat]

Never realised that this might be confusing, but I can see that this sentence helps. Thanks Adam!
Could you also generate the html (render_site()) so it will be displayed on the github.io page. (to my knowledge this not triggered automatically upon merge).

[ablack3]

Never realised that this might be confusing, but I can see that this sentence helps. Thanks Adam! Could you also generate the html (render_site()) so it will be displayed on the github.io page. (to my knowledge this not triggered automatically upon merge).

Sure I can do that. Just wanted to make it clear that the tables and fields need to exist even if they're not required.
Updates to the website files have been added to this PR.

[clairblacketer]

@ablack3 I am going through to clear some technical debt and am currently reviewing PRs. I don't agree with the statement that 'required' fields do not need to be populated. They all should be populated with non-null values, but any data that is not available, like race_concept_id, can be mapped to zero.

[MaximMoinat]

@clairblacketer The sentence that Adam has added "Fields that are not required should exist in the CDM table but do not need to be populated." I think we can agree with that.

[clairblacketer]

Ah thanks Maxim! I misread. I agree with the statement and I’ll pull in the pr

[ablack3]

I also recently learned that CDM columns can be in any order. Maybe that's clear to folks coming from a database background. I guess databases don't have a strict notion of column order.

[clairblacketer]

@ablack3 @MaximMoinat I am closing this PR because of some conflicts in the html files but I took the text and have since updated the website.