Add JSHint tags for code quality testing

Brackets commented with "E084" are brackets that shouldn't be there to comply with 0AD's code standards, but are required for JSHint to recognise the encapsulated `let`s correctly
 - Known P1 bug in JSHint, issue 1862, possible resolution in 1910
 - "E084" is the relevant JSHint error code that shouldn't appear
Add README and update mod.json