document use of post_id

[davewhiteland]

We have a convention on how post_id is used to handle things like "Nominated Member", "Womens Representative"... so document it.

From everypolitician/everypolitician#371:

In many countries, there's a distinction between people who were specifically elected to their legislative seat, and people who were appointed for various reasons.

There probably isn't a one-size-fits-all version that is strictly accurate, but within EveryPolitician consistency should be the primary drive, so that people can write generic tools without knowing the subtle differences everywhere, with strict precision an extra for people who need it in a local context.

I suspect, therefore, that we should simply create optional Posts on the Legislature for things like "Women’s Representative" etc. The organization_id should still be set on all Memberships, so anything that isn't aware of (or doesn't care about) the post_id can safely ignore it, but anything that needs to know more can resolve that.

Since csv-to-popolo 0.22, we've been able to include these as legislative_membership_type