Write documentation on how all date fields adhere to ISO 8601

[jungshadow]

Issue

All date fields in the standard are supposed to adhere to ISO 8601 (e.g. 2018-01-01), however, that is never spelled out explicitly in the documentation. The only mention of ISO 8601 is in the breakdown of the file naming convention:

${ELECTION_DATE} - The date of the election in ISO 8601 format.

Solution

Add section of the documentation that talks about the date fields in the preamble to the data field definitions.

[JDziurlaj]

I have a counter proposal (or maybe just an addition). Instead of mentioning ISO 8601 in the preamble section, create a new section, called Data Types, which would list the data types used and the expected format for each. This would give us an opportunity to discuss the use of array, boolean, etc. If this section gets too long, I might suggest moving it to the bottom as an appendix.

[JDziurlaj]

See #28

[JDziurlaj]

Resolved via PR #28