Update schema.md

Seeking to address https://github.com/project-open-data/project-open-data.github.io/issues/292.  

Note that this is accompanied by an appendix that is [still under construction](https://github.com/project-open-data/project-open-data.github.io/blob/metadata-v-1.1/iso8601_guidance.md#accrualperiodicity).